I’ve tried bi-amping a few times now, and even with the SAME two stereo amps I end up with a nagging feeling it’s robbing some of the musical coherence. Always end up going back to a single stereo amp in those cases. That was with solid-state amps, and horizontal bi-amping. I’ve also (briefly) tried with vertical bi-amp, and just felt there wasn’t much of an improvement to justify the expense.
Also tried bi-amping briefly on VAC tube amps - again same 2 amps each swappable from mono/stereo mode - and mono mode was so much better there was no point to stereo biamp.
If the 2 stereo amps are different, you would absolutely need gain controls (at least on the higher gain one) to match them, but even THEN I have to guess I wouldn’t like the result based on prior experience. So I think the problem boils down to: bi-amping with 2 of the same stereo amp doesn’t justify the 2x cost. bi-amping with 2 different amp is a Frankenstein approach, and you should just buy a single better amp.
I’ve had the Parasound Halo amps before (A23, A21, JC1) and the dual gain pots are nice to offset any L/R channel imbalance you have in your system. BUT that issue bugs me so much I prefer to address it in other ways (find the problem and fix it). But if the imbalance is due to acoustics (asymmetric room), THAT is hard to resolve and the dual gain pots could be your savior.