This is my first post here. Responding to Averageaviator (the OP).
I did exactly the same thing, including the propane conversion (GenConnex actually). These can run in parallel and the 2nd unit started will synchronize with the first. There are no magic cables or communication for this. It just senses the input from the 1st and synchronizes to it and maintains reasonable load sharing. BUT, thats 120VAC only.
I felt that it would work however to connect each generator to opposite sides of an autotransformer to get synchronized (out of phase) 120/240. Honda obviously would not approve, but it works for me.
I actually bought one of the Victron autotransformers first but it was not going to work because the two sides of the transformer were different by 5% which totally screws up the load balancing. It is not designed for this application - nothing is really. Most dual winding transformers seem to have a few "extra" windings on the secondary so that the losses show up as current, not voltage. So I returned the Victron and bought a Signal Transformer DU-5 which has 4 identical windings - and they were very, very close. With the windings paralleled, this can handle 41A of current which only really flows in the coils when you have a load imbalance. These two generators can just reach that if the entire 4400VA was on one side (unlikely, but possible). I used a 40A dual-pole breaker on the "output" of the autotransformer since in theory these two 20A generators together could push 40A on one side.
I would not try this with a smaller transformer - it won't handle the worst case load imbalance.
If you can find a surplus transformer out there with 2 identical 40A windings, it might be cheaper. I could not, so I sprung for the new one ($700 I think). This DU-5 weighs 90 lbs and barely gets warm with extended operation. The Victron's are MUCH lighter, probably more efficient and have a thermal shutdown mechanism. No idea how often that might trigger. I think both of them can only handle 25A in the transformer core (eg: imbalance current). But the winding mismatch was the deal killer.
I liked the Victron because it includes breakers etc. and is probably quite efficient in the right application, but it just did not meet the unique requirements for this application.
Why you might ask? Two reasons: I spent less than one Honda eu7700is would have cost (converted to propane) and I can turn off one generator and still have 120/240 and reduce fuel consumption. It won't start my well pump (2 of them will) but I can flip that off and run on 1 generator when I only need one. Obviously this is not enough power to be careless and we have to be aware of what we are running. I flip off the breakers for all the bigger loads that I know it can 't handle before I even start it up.
Oh, yes you could hook both generators to 1 side of an autotransformer and it would work and maybe even keep Honda happy, but then you always have current running in the transformer if there is any load on the other leg. I just thought it was snazzier to do it this way.