Combining Grid Tie Inverters and Off-Grid Inverters apparently can work, but there are some basic rules from what I've gathered... as this has been an area of keen interest to me.
For starters, it's best to use an Off Grid Inverter (pure sine wave only!) that's of the "low frequency" design... they utilize large toroidal output transformers, and can better handle "odd" conditions... such as overloads.
Side note: a Low Freq. (LF) inverter can usually handle 5x its rated load for "short term overloads", whereas a High Freq. (HF) inverter (also called "transformerless") can only handle 2x overload at best.
USA specifics: ALL inverters involved should be 240V split-phase (no 120V), to balance the whole load/grid or pseudo-grid. I would consider this a "must".
The Off Grid Inverter should be rated for 5 times (or more) of the total of the Grid Tie Inverters, to keep the Off Grid Inverter stable. This over-rating makes sure the Off-Grid Inverter is able to protect itself... so to speak. If you want to use 1kW worth of grid-tie inverters, better consider at least a 5kW LF off grid inverter.
Grid Tie Inverters will try to push power into your "grid", whether it's real or pseudo. However they have limits... they will not (should not!) push more than the "allowable maximum AC line voltage"... if you're in the USA, that's normally 252V (twice 126V, which is 120V +5%). Before this point they are supposed to shut OFF as a safety measure. (though I've seen some that allow the full "maximum short-term peak Voltage" rating of 264V)
I can't see a condition where this would happen, unless there was a no-load condition on your "local grid"... therefore you should probably make sure there's always some load, even by plugging an AC operated battery charger to run the excess solar grid-tie-inverted power back into your off-grid battery bank.
With an AC Line-Load sensing switch, you could enable a relay to connect the battery charger AC power to a "pre-sense" position of your local-grid under low-load conditions. (to prevent the low-load sense circuit from sensing the battery charger AC load, and turning it right back off... it should only sense "low local-grid load" conditions on everything EXCEPT the battery charger... confused yet? hey... it's almost 4am, I'm allowed to ramble)
The US Standard for Utility Power is ANSI C84.1 - but it's never been strictly enforced from what I can tell.
powerqualityworld.com/2011/04/ansi-c84-1-voltage-ratings-60-hertz.html
powerstandards.com/tutorials/VoltageRegulation.php
I can't find it right now, but I recently watched a YT video of one gentleman who had a 3-5KW off-grid battery pure sine wave inverter (NO outside grid on at the time), connected to multiple solar grid-tie inverters, and when he switched his solar array ON, the output load on his battery-powered off-grid inverter slowly went down to less than 10 watts output, but was still providing the "local-grid" with the 60Hz sine wave to keep the grid-tie inverters synchronized and running. I'll keep looking and post it if I find it... but I found these in the search:
youtube watch?v=R9uK_mDFzmM
youtube watch?v=GzCRKawk-Ag
OR...
There's a product called PowerRouter that supposedly takes on all these functions... but is likely very expensive.
powerrouter.com/en/powerrouter/howitworks-2.html
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