There's an elaborate procedure for putting a generator in sync with another. Traditional generators stay in sync because of load dynamics and backfeed forces (a generator a few degrees lagging will have its load eased, causing it to race faster and catch up). The problem is semiconductor inverters are absolutely oblivious to these forces, and will simply ignore them and bang their drums ever more out of sync until sparky smoky things happen.
I'm not a huge fan of using inverters generally in solar systems intended to run off-grid, as the overhead of the inverter is a total loss you must pay for dearly. I would be moving as many loads as possible to the DC side and see if you can put those on the smaller system and deprecate the inverter entirely. Ideally, only run the inverter when you have a non-DC-able load.
If you really must tie the two systems together into a common AC waveform, that's a hard problem. You could
- use DC as the shared language: tie them together on the DC side, with diodes, and both feed a single inverter. This of course depends on their battery voltage being equal and their batteries being of same chemistry.
- use inverter B's battery as the shared language: the smaller one's AC output feeds a battery charger for the larger one.
- use shaft torque as the shared language: using a magnetic rotary machine (motor-generator set) to sync them, either each turning a motor which turns a common shaft to a generator, or the smaller one having an M-G set, which you then manually sync to the larger one, the protective circuitry here would be an adventure.
- use water head as the shared language: install small hydro, and have both solar supplies backpump. It helps if you have a hill and a creek.
I know some of these solutions are pretty appalling, but these consumer tier inverters are just not made to do this.