vacs in series ( meaning the suction from one pulled through the exhaust of the other) will make the vacs spin at a higher rate, I don't know the exact figures but say individually each vac will spin at 4000rpm when the 1st vac suck through the 2nd both will spin at 6000rpm, so each vac is spinning 50% more, so obviously will wear out quicker.
if each machine is identical then the waste water will be evenly shared, but I think this would be impossible, so one machine will collect more water.
Mike