I'm trying to do a tab for a riff I'm working on, which has 2 guitar parts. One guitar plays in common time, while the other plays in 7/8. Powertab however, doesn't seem to allow 2 guitars to play simulatenously in different time signatures.

So my question is; is there any way I can change the time sig. for one staff, while keeping the other in a seperate sig?

It is impossible, sorry mate, but even a guitar book wouldn't show it like that...work around it, because as long as the parts sound OK then it doesn't matter too much what they look like on paper...
Yeah thats the thing, I wanna hear the two together, but have no means of recording, and no other guitarists about...ah well. Cheers anyway
Do you know how to go to the bass tab area?
If you do, set one(guitar or bass) to one of the signatures(4/4 or 7/8), then just pick the instrument you want.
set it to 8/8 and just make the one go into intervals of 7
so 7+1 , 6+2 , 5+3 , 4+4 , 3+5 , 2+6 , 1+7

8 measures of 7/8 would fit into 7 measures 8/8

or something.

I don't know in musical terms how I would describe that =\