Can I measure the Live-Live voltage safely with my multimeter?
Sure you can, but with the UPS oscillators not being synchronized a reading taken at any one instant will not tell you very much.
With no synchronization between the oscillators in each UPS, the phase angle between any two outputs will vary over time due to minor differences in the running frequencies.
Your meter reading will vary from zero volts (indicating in phase) to twice the nominal RMS output of one supply (indicating 180 degrees out of phase). Between zero and 180 degrees, you'll get a reading between those two extremes.
