Three windings transformer
If the outputs of two inverters need to be combined and stepped up to a higher voltage, that is probably a good way to do it. The frequency, voltage and phase relationship of the two inverters still needs to be controlled. The frequencies and voltages need to match fairly closely. The two inverters need to be in phase with each other or held to a specific phase difference. The phase difference could be used to reduce the harmonic content of the output waveform. If the transformer secondary is has a smaller windings of each phase connected in series with the main winding of another phase to form a "zig-zag" secondary, that can contribute to the harmonic reduction.

It seems like this must be part of a frequency changing scheme or a scheme for tying to grids together in a situation where the grids can not be coordinated with each other. Perhaps it is part of a power-outage ride-thru scheme. Whatever the case, two inverters may be used because there is something preventing one larger inverter from being used. However two inverters may be used just for the purpose of improving the final waveform.