|Are you asking what would happen if you plugged the UPS into itself?|
It would probably cycle on and off. When you turn it on it would see no voltage on the input and trigger the circuitry to produce power. The input would then see there is power and shut off the the circuitry to to produce power. Then the input would see there is no power and turn on the ciruitry to produce power, etc.
This would continue for a while but due to inefficiencies in the electronics, it would eventually drain the battery and shut off.