Looking at the stack closest to the viewer, the top cab is not even plugged in (at least not yet, if at all). I can only guess that they are only using one or two amps and cabs, and the others are backups that are super quickly swappable in case of a tube blowout or whatever.
Only need one or two if doing stereo, but who knows? Maybe the guy actually does run 12 amps at once.
I didn't see any cords going from amp to amp, but it's hard to see. If that were the case, I would guess that they are going preamp out to poweramp in, using one head's preamp with two heads' poweramps.
Those kind of cabs are single input, so only one amp is going to go in one cab. No two heads into a stereo cab here.
But if they are using a lot o heads at once, they could either split the signal after the guitar, or after the pedals, or after the main head preamp, and feed any number of amps/cabs. Lots of ways to do this, but totally unneccessary imo. Well, ok the literal wind and earthquaking feel of 12 amps at once at Malmsteen levels would probably be mindfu**ingblowing and I would never want to go back to one or two cabs ever again.
Btw, there are amps switchers for single cab use available, lots of things you could do. Sorry I don't know for sure how they're setting up, but after you understand the signal path, you can easily surmise the possibilities. Hope this helps, and yes you made me curious as f***. At 3:30 am. I'm hopeless. Jawa says goodnight.