It depends on the system. One of my inverter gensets runs on LP or gasoline out of the box. The smaller one does not. Both have parallel capability, and from the same manufacturer.Wattage is easier for folks to understand... but yeah.... phase and current, but within a programmed rpm range. They auto throttle down to sync and have set lower and upper ranges while in tandem operation. That makes me wonder if operating on LP, would it still be within that limited range of operation since operating on LP isn't part of it's normal programming.
Honestly though, I have no idea. Never heard of anyone trying it and never had occassion to test it out myself.
We had two diesel gensets in the Victory (52' motor lifeboat) and IIRC we could synch them. It's been 40+ years so I don't recall the procedure.