This is a demonstration on how I connected two UPS's together to double the output power capability and make a 3000 Watt sinewave inverter. There are some considerations in doing this which I do not cover in the video, and attempting this is a good way to destroy two UPS's/Inverters so think things through and be careful!
I used Smart UPS 1500 RM2U units, but this general information applies to all APC Smart UPS models and I believe it can be made to work with any APC Smart UPS product of this vintage.
Although no special precautions are needed to do what is shown in the video, this setup is capable of producing a voltage level more dangerous than that of a typical 240V electrical outlet and is potentially lethal. Please only attempt this if you are familiar with basic electrical safety or have experience working with dangerous voltage levels.
Basic Electrical Safety
http://www.labtrain.noaa.gov/osha600/refer/menu12a.pdf