Test the generators individually, and connected, by adding a load that is NOT your trailer, for example an electric heater or an old style 500 watt shop light. That will tell you if it is the trailer at fault, or one of the generators. This is how I load down, test and warm up my generator before I change the oil, it works very well.
I have never used a paired set so I have no specific comments on that part of the situation. Good luck.
John Davies'
Spokane WA