Don't automatically assume that doubling the batteries will extend the
run time. I was very surprised.

I've a 9 year old APC Smart-700 UPS. I tried at one point doubling up
on the batteries to extend run time. It performed as expected without
the powerchute software running - more than doubling the run time.

With powerchute, it performed as if the extra batteries were not there;
the run-time calibration 'couldn't see' the extra batteries, perhaps
because the EPROM 'knows' that no external batteries were installed.
I'd thought the hw/sw would look at the battery voltage (only) and use
that value to decide when to shut down; didn't happen.

You may get different results - different models, mfrs. firmware, etc.