A 101/2" reel of tape lasts about 15 minutes at 30ips and 30 minutes at 15ips, so the answer is you'll need several reels of tape

I don't recommend taping over any masters of an album. If your friend's band is using tape as an effect between the real world and a DAW, and they don't care what happens to it later, there is no reason not to re-use the reel.
Maybe I don't understand what you're asking.
If you break it down, is what you're asking different than- "can we record on used tape?"

One idea that is physically possible (although there are clear reasons not to do it) would be to use specific tracks for each band.

So, if your band is called SCOTUS and their band is called SCROTUS, you could use, say, tracks 1-4 for SCOTUS (ie. bass, drums, guit, vox) and 23-24 for SCROTUS drums (stereo). Then, what the heck, rewind and lay down more songs for SCOTUS, (5-8) etc.

This sort of thing is what you're getting at, right?

It is physically possible.

= Justin

yet another option is go to a studio with a stephens 40 track tape machine and both use 20 tracks.