I have to disagree with the idea that doing your own character pairs is superior to pairing up with other players. It's more fun to have your characters more closely responding to the dialogue and actions of other player characters rather than strictly following your own preordained whims. Plus, while the argument that strangers tend to drop out or disappear for long periods of time is valid, that's always going to be a problem that impacts the whole group, not just a couple of characters, and losing players is always going to be a problem unless you've managed to get together an RP consisting of just you and a dozen friends (and then there's the scheduling issues...)

Personally, I don't think having pairs controlled by two players instead of one is ever going to inherently hurt the speed of an RP, because it's not really addressing the core problem one way or another (that being: real life happens). And even if it did, what you might lose in speed you make up for in spontaneity and more dynamic character interactions, which I think is a fair trade.