I have a Harbor Freight Predator 3500 and an AiPower SUA2000id and have been running them in parallel for several years. I converted the Predator to tri fuel with a NashFuel kit a few years back. The AiPower is dual fuel and has only been run on propane since new about 2 years ago. I just use the front panel connectors with a 5 foot WEN cable kit from Amazon for $12. I have run other older gas only inverters with the Predator with no issues.
We had a power outage last week for about 4+ hours and as usual, I got my little cart set up in about 5 minutes and got back to my normal life. There is no trick or "Gotcha's" for running in parallel. Connect the cables and the other hookups prior to starting the first unit, then they work normally.
I tap the power output off of the Predator 30A panel plug going into a 3-4 lug adapter. That feeds a 12 foot 10ga. cable to an outdoor box I wired to my home panel. I have installed an interlock for the house main breaker, and 2 30A breakers for the panel input.
The gennys seem to balance their output to the load and the larger one takes on approx. 2/3 of the load from what the panel meter displays. I use a Kill-A-Watt meter to test with monthly, and that seems to be the norm.
This is not rocket surgery by any stretch, and if you do a little research you should be able to confirm and duplicate my results. I have plenty of pics for those who may be interested.