If you're experienced, I can't think of a weight difference that for me would be unacceptable. And, I've simul rapp'ed with someone I outweigh by 100 pounds. Also depends on the terrain and how you're rigged. Again, if you're experienced, then, there isn't a weight difference that would matter (IMHO).
A weight difference of 50% of the lighter persons weight is approximately the limit for most metal anchor points. Beyond that and you will probably just end up lifting your partner. You could still do it, but would have to go one at a time, kind of defeating the purpose.
Interesting anecdote: I Knew a guy who rapped off his end while simul-rapping, high up on a multi-pitch because his partner was saying " whatchyourend, whatchyourend, watchyourend," in a thick aussie accent, and he thought he was saying "watchout" or something, as in "rock", so he was looking up, and didn't notice the impending doom.