I don't know how 2 AC units could possibly run together on a 20 amp supply, even ignoring any other loads that could be on. If you are reading 15 amps on one, that'd be 2x15 = 30 amps.
Have you got both AC units connected to the same leg of the 50 amp service? I thought they were normally split so each one is on a different hot leg, unless your units are done differently?
Do you have specs on the AC units that will show what the normal full load current draw is (at 120 volts)?
Regardless, what voltages are you seeing?
