robin58,
Most small, and even some medium air compressors start across the line using contacts inside the pressure switch...just like turning on a light switch on the wall. If you plumb both compressor outputs into a common tank and install one switch to control both compressor motors, it will probably be too much load for the switch contacts. You need to make the pressure switch a control mechanism for another larger switch...a motor starter, that can be wired to start both compressors at once.
Plugging the compressors into separate circuits is easy, but you still need to close some contacts (pressure switch) to start both motors.
One pressure switch can handle the current to start one motor, but if you hook the second motor to it, you might overpower the little contacts, electrically speaking. If you look on the pressure switch, you might see a horsepower rating as well as a current rating (amps). If the combined HP of both electric motors doesn't exceed the rating of the pressure switch, you are all set.
This all sounds very anal, but it is the approach you need to take on something like this. The other method would be to stick all the things together and see what happened. This has been know to work just fine too.
Motor control stuff can be difficult to understand...why can't I just plug it in? You won't electrocute yourself, and with the circuit breakers you are pretty safe. Dink around and see what develops, then let us know.
Stuart
