Hi Claudio,
I'm not a performer, so I can't really advise. but you could use
VA7 to play styles in realtime or as a sequenced song/backing track, which pretty much takes care of the backing,
A motiff could be used for a sequenced song/backing track, but remember, it doesn't have styles ( like the VA76) so you'd have to record these songs in your sequencer from pretty much scratch.


You or your friend could use the casio to add additional parts to fill the backing out ie add strings, guitar whatever ie you're using it as a synthesizer.

I don't think you've mentioned whether you have a singer or whether one of the keyboards is going to play melody part?? If you're using keyboard for melody, then either the VA76 or Casio or Motiff could be used. Motiff would probably have the better set of sounds as it is a dedicated synthesizer.

If anyone can saw any flaws in my logic, please jump in, I'm not a performer.

Years back I've seen bands where a keyboard player had 3, 4 synths stacked together.
I would imagine the only reason for doing it is to give the player a choice of variety of sounds. Each manufactures synths had their own sound. Some had better piano's some had better strings, some had better brass, but it was probably in the synthesized type sounds that they varied the most.
The quality of the modern day synth & keyboard has come a long long way.

best wishes
Rikki