Basically, you could use two V4s, but hook the second up to the first from the EXT. AMP out of amp #1 to one of the EXT. AMP inputs of amp #2, by passing that amps pre section. You'd then control both amps with amp #1s controls.
I did this at a crazy drunk house show a little while back, each amp going into its own 1x15:
For the recent stuff my band has recorded, I've been using the EXT AMP out into a DI along with a mic signal, and it sounds awesome together.
This would double the wattage, yes? Does it HAVE to be another V4?