I did it for a college project once. It was fun, mostly because we were building something we already had defined, so it was just a matter of implementation and mutually learning the environment (we were using Max/MSP). It probably worked because we were in unfamiliar territory; if we both knew the lang inside and out, I bet we would have just bitten off a bigger chunk and each done our own portion and evolved it further.

I don't think I could do it on a significant project, because I'm always making a bunch of tiny changes that I wouldn't want to have to explain/suggest. Collaboration on a single problem is helpful. Collaboration on a single work of art isn't always. Could you imagine pair painting?