ok heres the deal with the ups, if by some stroke of luck you can find a single 12v battery with the same AH amp hours and the same discharge rate ie 500va 650 va etc. and that has the same charge cycle as your 2 6's then you'll be fine. heres the issue, if you there is more load on the battery now and its not rated as high as the 2 batteries running in series, you could cause fire or loose the effectiveness of having clean power comming into your system. also the charger circuit in the ups is looking for its oem batteries and will not adjust how it charges for a different type of battery. so if you overcharge the battery all the time, the acid burns up and your battery looses its juice very quickly, or if it was undercharged all the time, the discharge time may be slower, loosing the effectiveness of the ups. Best bet buy new oem batteries lol