Based on posts that I've read, the Yamaha generators have a more standard 120 volt output as compared to the Hondas that tend to run about 128 volts until heavily loaded. This means that if the generators did sync ok, the Honda would carry all of the load until it was close to overloading. Only then would the Yamaha pick up part of the load. The output voltages are incompatible, and they would not make a good combination. I've not read of anybody actually trying it. Theory is nice, but even if the risk seems small, most or all of us are too scared to take the chance of burning out an expensive generator.
