The MH is 50 amp.
Doug - I swapped the black cables (though one was actually black/yellow, which gave me pause. I researched, then felt comfortable swapping them). I then repeated my troubleshooting:
With only 20 amp CB off:
Both AC OFF
All outlets ON
All lights OFF
With only 30 amp CB off:
Both AC OFF
All outlets OFF
All lights OFF
When I try to run both AC at the same time, the 20 amp CB trips, which is what it was doing before.
Doesn't seem like each AC has its own generator CB, unless I'm misunderstanding things. Otherwise one would work if at least one CB was on, right?