I had to temporarily do this while a unit was out of service. The system could not start up with only 2 inverters, even though continue with missing phase was selected and number of inverters was adjusted. Thread here: Continue with Missing Phase is Missing the Point
The way I did it was to set both up in standalone mode. Both followed the grid and stayed 120° apart. In the event of a grid failure, they would not stay synchronised and will inevitably drift. This was acceptable to me, as the neutral cable was sufficient and protected for the full load of 2x inverters.
I only ran like this for week while the RMA was in progress, until I reverted to full 3 phase operation.