I suggested running a second game to one of my players if I could find the time and they mentioned just alternating it with my Black Crusade game. But then it came down to what I would run.

What would tips and tricks be to include a Space Marine, A Rogue Trader character and then a Dark Heresy character. What would the ranks be for each if the game was at start? Any other helpful advice?

 The rogue trader character, especially if he is a rogue trader fits easily enough. He is powerful in another way than the marine and able to bring the armies to bear against his enemies.  So just handing him some xp and a profit factor and  he should be good to go, but do remember that he has a dynasty, can buy pretty much anything ect.

A dark heresy character represents another problem, he needs to be created using the ascension rules which opens up to the possibility that he might be an inquisitor and then he's suddenly almost per default in charge. Assassins are pretty powerful and so is the Ad mech,  Primaris Psychers and sisters of battle, so they could work as well. Guardsmen and arbites would probably feel a little useless, depending on the game a scum might be a lot of fun or an adept.

Thanks for that! Though I was meaning more rules wise.

A level 1 Deathwatch is roughly equal to a rank 9 (ascension) Dark Heresy character and a rank 3 Rogue Trader correct? Also how to handle how each of the games handles rules for Psykers/Librarians? Armor and weapons are pretty much done the same.

Another question of mine is how should DoS be handled. Use the updated one in Black Crusade were it starts counting from zero or to continue with the original were the 1st DoS comes after 10. If using the Black Crusade way does anything drastic change?
 

I believe the technical answer for the psychic powers (and really the rest of the system) is that a "dominant" system needs to be selected. What book are you looking to ultimately for combat rules? From there, everyone uses those rules/talent descriptions/traits/skills from that book, but uses their respective books for character advancement (xp costs, talent/skill/trait availability, etc.). Gear fortunately is pretty constant among the 3 books, so is less of an issue.

Fortunately, once you start using Ascension rules, almost all psykers follow the fettered/unfettered/pushed rules, so really there isn't much in the way of rulings.

As far as DoSs are concerned, this falls under the selection of the rules you wish to go with. The big question once again is what is the main system being used here? you could certainly go with BC if you wish (Only War's beta rules could make for an interesting game). I must warn you, enemies tend to be built with the governing system in mind.

For example, a DW Bloodthirster has a WS of 99. It is built with the idea that Lightning Attack is only 3 attacks. If you use the BC/OW system, the Bloodthirster in theory could pull off about 8-9 attacks in a half round action. This is unhealthy for a party (of course, 3 attacks could also very much stomp a characters face in, at least the 9 are made against a single dodge action).

Here's a tip: don't do that. The balance will be all over the place.

Here's a more constructive tip: buy Only War beta while it lasts. Convert existing careers from previous games to the new Aptitudes system, calculate the cost of all the advancements they posses as part of their starting packages to determine the exp value at which the career starts the game, then hand out the same amount of exp to lower level careers to spend as they desire. BAM, you have balanced characters of different backgrounds in a few afternoons of work.

 The Vindicare Assassin is fully capable of tearing a Kill Team of three Marines into small, bloody ribbons. The Primaris is so mind-bendingly powerful, he doesn't have to, because the Kill team couldn't hurt him without the specialized anti-psychic weapons, and he'd simply flick his hand and use Psychic Crush or something to turn the offending marine into a snail beneath a boot. Crunch.. The systems are designed to be "compatible" but Dark heresy has fundamental differences that make it unrealistic, most notably the Psychic system. Changing from the multiple d10's + WP bonus to the WP test destroys, imo, the power of a Psyker completely, it just makes it safe.

Not impossible, but be very, very on your guard for abuse of power, because the potential is there when you have some 9000 odd experience to spend on abilities a Marine couldn't get for another 6000.

 I've already said no too Vindicare Assassins and Primaris Psykers… so far I have a Sister of Battle, a Magos, a Sphess Mahreen, a death cult assassin and two other characters that have yet to be decided.

What about another approach: Don't equalize the power levels at all.

Space Marines should be recognizable more powerful than any other character class – after all, they are genetically modified superhumans. This important part of the 40k fluff would cease to exist if you'd raise the other character classes on Space Marine-level. 

I'd suggest to rather create a balance by assigning specific roles and competences in your adventure: The Dark Heresy character would be powerless compared to the others, right. But you could give him the command above the Space Marines: He's the one who got the commandership for that mission by his inquisitor; the Space Marines are the troups that have to do the bloody work. So the least powerful guy would still be fun to play as he's the one who determines where the story goes. 

The Rogue Trader could be the one who supports the Inquisition, e.g. by transporting the acolytes and Space Marines. It would be fun to watch the Rogue Trader and the Acolyte (who has the commandership) discuss who of them is the real boss.

Another approach:

Give at least two characters to any of your players: an Acolyte and a Space Marine. Then you switch between both character classes: At first your players are the Acolytes, investigating what mysteries have happened. As soon as the enemies are detected, your players switch to the Space Marines who are ordered to clean the area. How cool would it be to just throw a drop pod next to your Acolytes feet, letting the Space Marines declare that they're taking over the mission to get the job done. And then you tell your players that THEY are now the Space Marines…

I'm one of the players in question. The Assassin (Death Cult, not Vindicare) specifically.

Giving me a level 1 Dark Heresy character, and asking me to hang out with a Space Marine and a Rogue Trader and go on missions with them is ridiculous. I'd have 4 stats at 25, and a boot knife, and the Marine would have a 55 combat skill and do 1d10+23 with his chainsword. How is that even an idea? Not to mention my Rogue Trader Arch Militant (The class from RT the other player wants) has a BS of 69 at Rank 1, which means I might as well not even play if I am level 1. Again, its not just combat either, his characteristics will be higher than mine to be enough that he is better at all skills than I am, too.

Not to mentiion his untrained skills equal my trained ones. I don't want to play a space marine at any point, so I don't see what value there is to having 2 characters, unless my second character is a Kaballite Warrior. That I could work with. Space Marine? No thanks.

My thought would be to pick a combat system, like Black Crusade and use it. Then equalize xp, RT is supposedly around 8000 and Marines around 13000, so I'd give the DH characters like 9500 or so xp, and the RT's like 1500, give out reasonable gear to the RT in addition to the free aquisitions, give out whatever gear a Rank 7 DH character should expect to have,and limit the Marines gear to whatever he starts with + signature gear talents and anything he loots.

It might not be exact, but I think its close enough for both a game, and government work.

 Removing a Deathwatch character's ability to requisition wargear per mission would limit that character. Just saying.

I'd say being 4500 xp behind the character limits the others, too, so them being unable to requisition freely might be a fair price to pay, no?

 Not really, since the Marine is paying a lot more for his advances than the others. Unless you give out differing XP to characters based on their game, the Marine is going to fall behind on talents and skills sooner or later.

He'll never fall behind on characteristics, since they start out almost as high as other people can get through 4 advances.

 Which is fitting, considering the others are humans, whereas he's been genetically manipulated to be superior. My point was that the balancing factor for Marines starting a few thousand experience above the other characters is that he would have had to spend that extra experience to get to his starting point anyway, and that he's going to continue to spend 1000 experience for a skill advance that costs the others, what, 300 in Dark Heresy and 500 in Rogue Trader?

EDIT: Editing because I remembered another point that's applicable. The argument for Marines starting with superior stats meaning they should be penalized elsewhere for balance works in Black Crusade because the Marines start better and they pay the same cost as humans for their advances. With a Deathwatch character in a Rogue Trader or Dark Heresy game, that doesn't apply. It's rare for a Deathwatch character to spend less than 1,000 XP on an advance in anything- characteristic, skill, or talent.

Well, with better starting everything, including gear, I dont see any reason to make the humans pay a resource for gear also, and then give the marine whatever the marine wants, either.

I mean, I want everyone to feel like they are getting a good deal. The Marine gets better stats, better skills, better gear, better armor, unnatural characteristics, it seems fair enough to me that they cant then get whatever they want for gear before every single mission. It isn't like we're on a Chapter ship, with an Artificer standing around to hand him stuff (at least, I dont think we are…)

I don't see where you feel the balance comes in, unless you mean "7000 xp from now, the Marine will only have a few more talents" Because there is never a guarantee any game is going to go that far.

 Since you're not going to listen to that argument, lemme try another. Marines don't loot. Ever. No loyalist marine is going to pick up a Chaos Marine's plasma gun or a Tau rail rifle. It doesn't happen. Marines can't buy gear that fits for them, either. If it's that big of a deal, set the requisition relatively low and give the Marines small amounts of Renown. Charging the Marine experience to get anything beyond their basic weaponry isn't fair to them.