Please explain two in parallel??
Unless you are using one of our APC auto xfer switches you can use two in parallel.
Also do you have anything else plugged into your 1500? UPS are rated in VA which is not Watts when you calculate power factor.
I ran two boxes, one with quad SLI (1000W PCPC PS) the other a single 8800GTX both with LCDs and never came close to pushing my 1500, however mine is a smartUPS XL.
BTW I work for APC