I don't know anything about Learjets either, but I would have thought that any system with more than one inverter (or rotating alternator) could have problems when they get out of phase. If they are paralleled on a common "bus" a small out of phase condition would lead to higher loads on each inverter and harmonics on the supply. As the phase differential gets worse it would eventually lead to both machines tripping or even failing completely. You are effectively connecting +110 volts to the same bus as -110 volts!
If they were supplying completely separate systems this would not arise, but I assume the normal arrangement is to have them paralleled for redundancy.