Diagnosis
Output synchronization failure. One generator's inverter board is detecting a phase mismatch and shutting down for protection.
How to Fix It
Start both generators separately first. Make sure both are running steady with Eco Mode OFF. Plug the parallel cables into the *generators* first. Then plug the cables into each other. Finally, turn on the parallel switch on *both* control panels. The order matters.
If That Didn't Work
One of the parallel ports on a generator might be damaged. Inspect the pins inside the port for bending or corrosion. A bent pin will prevent proper communication.
Heads Up
The generators must be the same model (iGen4500 to iGen4500) for parallel to work correctly. Even a different year model can have firmware differences that cause this.