I think that I've seen "stuff" about using the current
along the cycle to keep the inverter output slightly
leading the mains-reference, in grid tied inverters,
so as to always be delivering energy into the
waveform.
But who's in charge when you have two identical
inverters with no master, is a tricky bit.
I see inverter generators with parallel capability
but they want a special expensive cable. What's
so terrible about imposing communication on
the two inverters? You're already running 3 wires
(hot, neutral, ground), right?
Seems like an opportunity for "power line
communications" protocols, to throw sync and
demand messaging.