Can one parallel two identical batteries and connect to the UPS, and will it act as a single battery of double the capacity? Or is there a problem electronically with it?
According to this answer, and commonsense battery knowhow, the only thing that matters technically about a UPS battery is that it's got the same (or compatible) chemistry and voltage output, and an appropriate charge capacity (VA rating).
However the latter's only effect is probably how quickly it runs down. I'm guessing at basic requirements - they run completely in parallel and they have similar capacities so that we don't get charging loops between them.
I'm also guessing that it's much like any other issue of parallel batteries of identical characteristics. SO if there is a problem it's one that basic electronics can handle - maybe diodes or other rectifiers? But not sure what.
Yes it's not what manufacturers would recommend. No that doesn't mean automatically it's a bad idea :) It might be a bad idea for other more serious reasons though.
Can it be safely done? if so does it need any other circuitry to ensure it goes well, and if so what's needed?