I'm not sure whether your GFCIs incorporate normal overload protection as well. Over here most don't, but a few models have it.

Are both motors actually starting simultaneously? The inductive load could cause some types to trip in this case.

One thing I would check for straight away is a neutral-to-ground short on the circuit. If such a short exists, it could be the the parallel path isn't carrying enough current to trip on one motor, but is on two. I've had that sort of problem several times.