They could always get back to their own mojo after their joint master-work was released. And, yes, they'd have to be very open-minded and cooperative to ever pull it off.
On the Steven part, I can see him putting his talents to an cRPG idea I've always been intrigued with: NPC's that are true rivals/equals to the player. NPC's that roam the world and adventure and perhaps compete with the player for quest rewards, faction favor, loot, etc. Maybe even a plot-line NPC or two that actively work against or to the benefit of the player even when the player isn't directly interacting with them or even near them. And it would have to be dynamic, like DoP, rather than strictly scripted. Wouldn't it be cool to stumble on your arch-nemesis by pure chance in a town and have an old fashioned Western showdown in the street while the townsfolk scurried for cover? Then you find that key artifact on him that you had been looking for to continue a side-quest you'd had to abandon earlier? Or think of a Belock(sp?)-Indiana Jones type relationship where you're always going back and forth with each other throughout the game stealing stuff from each other and trying to get the other guy killed without directly doing it yourself, because it's too politically sensitive or morally hazardous.