Within limits - yes
It is hard to use a completely conventional off grid inverter for this because they cannot sink power. Something like a Trace SW or an Outback GT inverter is a better idea since they will sink and source power, and thus can absorb some of the 'excess' power into the battery bank. This is called AC coupling. You also need to be able to regulate AC power to the inverter, something that a straight grid tie inverter can't do.
Here's a paper on the idea: http://www.outbackpower.com/pdf/tech..._WHT_Paper.pdf
What is the interface? Just tie them together at the load side of the AC panel and the slave (conventional grid tie) syncs to the master (off grid battery backup inverter) and shuts it self off if the master goes down because of low battery voltage?
Here's a paper on the idea: http://www.outbackpower.com/pdf/tech..._WHT_Paper.pdf
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