Broome101, I agree with Dave on being able to run both A/Cs at the same time - as long as you are on 50 amp service. Otherwise, I would it would defeat the whole purpose for putting in the 2nd A/C in the first place.
Dave, How about weight difference, power pull difference and noise level difference? I know that some A/Cs are much quieter than others - especially on startup. And I thought I had heard somewhere that some of the reason for noise from an A/C is in the ducting (although the 2nd A/C has no ducting).