The two circuits may be considered the high / low = on the pressure side.
If you set the PA temp selector to 60 F and push the Auto button as well the compressor clutch should engage.
If it does not engage it generally is that you have a leak and have insufficient refrigerant.
If the clutch does engage the accumulator should get very cold quickly.
If you want a good read just google:
"Auto air conditioner blowing warm air not cooling"
Hope this helps !