Can I parallel up two of the same UPS units?

It must be possible to parallel up to UPS units of the same type & Wattage?

I did it it with transformers, too. Of course one needs to get the correct wires together, according to the direction of the winding/flux.
I can understand that two unit of different output cannot be paralleled up, because the total output will be limited to the smallest unit's output, like with resistors.

A UPS has an AC output. Two will have their oscillator frequencies slightly different resulting in one having a negative momentary output and the other having a positive momentary output. The load will get nothing but the inverters will blow up.

Mains transformers operate at exactly the same frequency.
Maybe you can sync one UPS inverter to the other.

I agree with Audioguru. Without a synchronizer you WILL blow up your UPS equipment.

They will blow up if they aren't designed for it because the won't be in phase with each other.

Some UPSes can be paralleld, one is set as the master and the other is set as slave. The slave sychronises itself with the master and suplies the same output voltage and current.

Check the datasheet, it normally isn't as simple as paralleling the outputs, you sometimes have to connect a communications port or something else.