Inverters in parallel. Forcing one to be in phase with other?
Both inverters have their own voltage loop for regulating the output voltage (very likley but there are obscure examples which are entierly open loop). If you don't have the loops in unison/one loop for both, you end up with a simular sutuation to two thermostats in the same room, one for heating and one for AC, with both fighting each other. That's why you need to join them. 

If the inverter shuts down at overload, you do not have a constant current limit. Without this functionality it will be nearly impossible to make them work in paralell unless you redesign the control circuit in both from scratch, for both CC limit and one loop for both. If you do pull that off, I would be very impressed and you should have no problem finding a job in the power electronics industry.

TL;DR: Buy a 1600 W inverter!