Sounds like you have an Energy Management system by Intelletec.
Or there is a switch somewhere you can not find
here is how it works
The Auto Transfer switch has two transfer sections. The Generator has two circuit breakers (30 and 20) (usually may be 30/30 but then ,,,, )
You did say you had a 50 amp rig (Generator likely 30/30)
The intelletec EMS only lets one A/C run at a time unless it either gets teh Generator signal (Turns 'em both on) or sees voltage (240) L-1 to L-2 (If you plug into 20 or 30 amps using an adapter the l-1/l-2 voltage difference is, ideally, zero, in practice not much)
There may be a "Front/rear/both" switch somewhere (Both only works on Generator or 50 amps)