Several GMs get together and muster a bunch of players. There seem to be enough out there and I think a Mega campaign would be fun. Start with a Rogue Trader party comprised of the most active players and the ones with the RT book. Have that party work through endeavors. Once that game is rolling, another set of players could play through part of the endevor in the Dark heresy system.
ex: The Rogue Traders want to set up contracts with the Cold Couriers Guild. So they send in some men (the dark heresy party) to locate and persuade the guild. The Rouge Trader makes the profit or command or charm check(s) and continues on to the next leg of adventure. Meanwhile that part of the endeavor falls to the Dark Hersey party, where they must hunt through the underhives, avoid assassin and pull off a high profile jail bust to secure the Guilds trust and support for their rogue trader leader.
All the while, the rogue trader game is being played through.
After the Dark Hersey adventure concludes the Rogue traders will invariable come up with a whole new slew of scenarios for the Dark Heresy Players to become embroiled in. There could even be crossover between the to systems.
Comments and anyone who wants to try?