I think Maybe... Three bands come to mind...

Band No.1:

Barret: Vocals
Jessie: Guitar
Biggs: Bass
Wedge: Drums.

Hmm, they could call themselves AVALANCHE, since that could probably work as a good band name.

Band No.2:

Reno: Guitar; Vocals
Vincent: Bass; Backing Vocals
Cait Sith: Drums
Kimarhi: Rythm Guitar

Erm, I was picking characters in this one, seemed like i've already used too many FFVII characters, so I put Kimarhi in for the erm... sexiness? x.o

Last band:

Vincent: Lute
Red XIII: Ukilalei
Blank: Banjo
Kuja: Triangle
Steiner: Maracas
Seymour: Xylaphone
Sephiroth: Digeridoo (It's long, like his sword )
Barret: Backup Maracas (Incase Steiner brakes his...)

That was just the stupid band, but I suppose this would probably be the best one of the three. But seriously, Irvine and Vincent playing a duet Acoustic set in some old, dark night club somewhere, that would probably be pretty cool.