Here's what I have found so far.
I have two strings of 5 250w panels, I see no grounding issues and they each read approximately the same voltage output with a meter.
The strings go to a dual PV isolator switch mounted near the inverters and from there one string to each pv1 input on the inverters.
Connections are tight and the wiring at the switch to the inverters is correct.
If I run the inverters on battery only the 120V output is rock steady, if I connect the solar switch providing solar input to both inverters at their pv1 inlets, the output voltage swings all over the place on both inverters.
Now, If I disconnect one-panel string, It doesn't matter which one or which inverter and run it off one set of panels everything is fine. I have tried this with both inverters one at a time and both strings, to eliminate an inverter issue.
So, one PV string to either inverter, either string or inverter, it works fine, connect the second string of PV and the output is all over the map.
One odd thing I have noticed is that I have 32 volts at the output side of the Isolator switch even with the panels unplugged, it's coming from each inverter PV input line. I don't get that.
Both inverters are wired back to a transfer switch in the main house panel and bonded there only, that bond is connected whether or not the xfer switch is closed as it just closes L1 and L2.
And both inverter screws are still removed maybe this?