A 5k Onan will run 2 AC units. Your 6.5 is MORE than capable of running 2 AC units. Since the 2nd load kills the APU, try running the 2nd AC FIRST and then start the 1st unit. IF the APU still dies, your APU carb/Governor system is not set up correctly. OR your APU is NOT running at the specified 1800 RPM's. The Governor keeps the APU AT 1800 when loads are applied. When you apply a load, the Governor "pulls" the throttle slightly to keep the APU at 1800 RPM's, and constantly adjusts depending on the load. The odds are the Carb is gummed up. Doug