With grid disconnected, Sunny Island will ramp up frequency until power production from GT PV drops to desired level. I saw it go as high as 64 Hz or so when GT PV didn't curtail output.
Rule-21, UL-1741-SA, or whatever should work so long as "frequency-watts" is implemented. That may be an optional item to select (earlier implementations were supposed to remain on-line at full power for 299 seconds, drop off if frequency remained high for 300 seconds.)
When my loads are about zero, frequency hovers near 62 Hz, which is where Sunny Boy output drops to zero. For Rule-21, I think that would be 61 Hz. I don't know whether inverter would remain connected with zero watt output at 61.1 Hz, or disconnect. Possibly zero load would flirt with disconnction.
Default Sunny Island configuration will also go down to 59 Hz to get mechanical clocks & timers back on track. Again, possible 58.9 vs. 59.1 Hz would make a difference in knocking inverter offline. You can configure SI to not make those low-frequency excursions.
2x SI (12kW continuous output) can manage up to 24kW GT PV if never grid connected. But it can't suck that much power down to battery. Around 12kW or 14kW, it could handle 100% load dump and put the power into battery while shifting frequency over a few seconds, but only if battery accepts the current. (100A continuous per SI, 140A peak.)
If you have 14kW, that slightly exceeds 13.4kW max allowed on-grid, 240V x 56A relay. Your results may vary with line voltage.
If you put one 7kW (7.6kW?) Solar Edge on protected loads panel downstream of SI, and one Solar Edge on main panel not downstream of SI, that would avoid overloading the relays.
If you put an interlocked backfed breaker on main loads panel, then you could connect both Solar Edge to output of SI, only when not connected to grid. No issue with wattage or relay max current in that case.
I have ac coupled the Solaredges with the Sunny Islands in a critical loads panel. I still need to change the Solaredge settings to California 21. Unfortunately you need installer rights to have access to those settings, So I'm working on that right now. Once I have California 21 enabled, I'll run a test by disconnecting from the grid in full sun to see how it goes.
What would be sign of first trouble? The Sunny Islands would keep recycling/restarting every 5 minutes if the solaredges aren't throttling down the PV output. Does that sound right?
Sunny Island will keep running. It will raise frequency possibly knocking Solar Edge offline for 5 minutes (depending on how Rule-21 works.) Battery will cycle as PV produces, then doesn't.
Do you have a meter that reads Hz? My Sunny Boys only display that when out of range.