Hello! Newly built system is a little finicky.

[dadboy]

I recently put together a system with 12, 385 watt Phono Solar panels, 4 SOK 206 ah batteries, mated with 2 MPP Solar LV6548 inverters. It push power to a Square D Homeline panel. I have a few questions that I can't find the answers to. I have the both inverters wired to charge the batteries at the end of the day. When both are plugged in, the breaker panel loses it's 240V output. When I unplug one, the 240V returns. Doesn't matter which inverter I pick to plug in, 2P1 or 2P2. I thought maybe I wired it wrong. Maybe I missed something in the setup? Next, when checking the PVI input, volts seemed ok, amps was always under 1A and watts was in single digits to low 20's. Once again I wonder if I did something wrong in setup.
Finally, if I use WatchPower, does it matter what inverter I plug into? Panels are in 4 strings, 3 panels in each string.

 

[Hedges]

Fighting each other? Tripping a breaker?
Or just 120V - 120V = zero volts?

You say "plug"; normally inverters are hard wired, not plugged in.

How about photos and a diagram showing two inverters, breaker panel, details about where L1, L2, N of breaker panel wire to whatever terminals of each inverter.

Checking voltages between wires when they are not yet connected, and connecting through an incandescent light bulb, can be enlightening.