DIAGNOSIS This is a classic synchronization issue. For parallel operation, both generators need to be in near-perfect sync for voltage and frequency before you connect the parallel cables. If they're too far off, one will see it as a fault and shut down.
FIX Here's the proper sequence: 1) Make sure both generators are OFF. 2) Connect the parallel cables firmly between both units. 3) Start the FIRST generator and let it run for a minute to stabilize. 4) Start the SECOND generator. 5) On the second generator, press and hold the "Parallel" button. You should hear its engine speed adjust slightly to match the first unit. 6) Once they're synced (the light may come on or the sound will stabilize), release the button. Both should now be running and sharing the load.
IF THAT DIDN'T WORK Check the parallel cables and ports for damage. A bent pin in the connector can prevent proper communication. Also, ensure both generators have a similar load level on them—trying to parallel a heavily loaded gen with a no-load gen is tricky.
HEADS UP Never connect or disconnect the parallel cables while either generator is running. Always turn both off first. You can cause serious damage to the inverters.