Generally, paralleling the batteries is not a problem. As the other
poster pointed out, the charging time will increase. Should you
experience a power outage and run the batteries down, it will take
longer to get back up to 100% charge, but for most locations with
"typical" power quality, this set up would be just fine. One important
note on this subject is to not let the batteries drain down to less
than 20% charge. These are sealed lead acid batteries and should be
thought of as a mini car battery. We all know that if you leave your
lights on and drain your car battery more than 6 times in the life of
a battery, you permanently affect the lifespan of the battery. Most
UPS manufacturers have a setting in the software to set the minimum
battery charge percentage.

R. Jessen
ReplacementUPSbattery.com