I have twins also-- lots of them, actually (well, okay, just four)-- and am well familiar with the "this half of the world is his, and this half of the world is mine" phenomenon. I can only pass along that my eldest-by-two-minutes is now, as an adult, regretting the fact that she gave her sister custody of math.
I kind of learned with the younger two-- they're allowed to claim ownership (or non-ownership) of extracurriculars and electives. One is a band geek, one is in theatre; one's in Spanish and one Chinese. But when it comes to regular academics,they are strongly discouraged from using "but I don't like it/that's his thing, not mine" as a rationale.
