They're littermates, so they've been squabbling all their lives (they're 2.5 years old now), but it seems to have become more meaningful in the past few months.

They can't eat out of side-by-side food bowls anymore; the big one shoves the small one aside. And there are nasty hissing and chasing attacks, though mostly what gets damaged is anything in the house that's stacked, like CDs or books. They don't seem to injure each other. If it weren't for the peeing, I think we'd all be in an ongoing state of detente with occasional squabbles.