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