We don’t have to work alone when we’re coming up with our PCs’ histories—and a lot of us don’t want to! GMs like cross-backstories because it means they can circumvent the getting the group together part and focus on the actual plot; a lot of players enjoy the chance to collaborate—heck, some game systems even flat-out insist on everyone having at least one backstory connection. For the most part, they encompass the same sorts of skills and techniques that a standard character backstory does, but are complicated by the matter of requiring two different minds to make them work. Here’s one process I found works pretty well for putting together a shared backstory between two players.
First, figure out what sort of social dynamic they should optimally end up with. Are they war-buddies, rivals, relatives, mentor and student, lovers, enemies, something else? Is this a long-time connection, or something more recent? How well do they trust each other? This provides a good framework for the rest of the shared backstory; if stuck, all you have to do is remember that the story needs to justify this connection.
Now, in the vaguest possible terms, agree on when and how they met, and whether anyone else was involved. Right now you’re looking for something simple: a main idea, if you will. The kind of circumstances and environment that would get these characters together. For example, in the time I’ve been doing cross-backstories, I’ve had a pair of school buddies in an ultimately self-serving alliance based on their shared interest in spywork and double-dealing, one somewhat messy romance, one mentor-student relationship enforced by external factors, one pair brought together by one finding a stopgap solution for another’s major problem, one master/creation duo (it was an odd one-shot, let’s put it that way), one spirit/favorite mortal pair, and my latest, involving a pair of con artists whose prank war caught the attention of a local semi-retired spymaster). When starting out, make it a bit vague—the main thing to make sure of is that the kind of first meeting you’re looking at would, in fact, get the characters’ dynamic going towards the result you want, and that whatever happened is actually possible for the world.
At this point, there are two directions in which you can go. One of them is to keep working on the backstory, bringing it into focus—what’s the place they’re meeting in like? If there’s another character involved, who or what are they? What sort of sequence of events made up the vague summary you’ve got so far? What about what happened keeps your characters together? On the other hand, if you don’t have time right now, or if you’re both stuck, this can be a good time to separate for a little while and work on the parts of the characters’ backstories that aren’t bound up together. Not only will it give your subconscious time to chew on the shared parts, but sometimes, something you come up with in the solo part of the backstory will explain, or at least break a tie between two options for, something in the shared part.
From there, rinse and repeat, alternating focusing and solo plotting until the result satisfies you. Enjoy!