Having had many mysteries myself, I have found that the simplest answer is usually the right one. If you can run 2 AC units at the same time you obviously have a generator that has 2 circuit breakers on the generator itself. Remove the generator cover and look for 2 'light switch' type switches. With the genny off, flip with switches off then back on. This may not fix your problem but it is easy and free to find out.
I can't tell you the number of times I was "sure" of something to only have to say well I "thougt" it was. The mind is an amazing thing and we can often fool ourselves in trusting our memory. That's why I always say "check the simplest things first".
Good luck!
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