robin,
Both compressors would be plumbed into a common tank with only one pressure switch controlling both pumps. The switch would simply be set at the maximum pressure desired, just like any compressor. What you gain from this setup, if it would work, is the combined output of two pumps in CFM.
I don't think this would be practical for a home shop as you would have to modify the electrics to control and start the compressors at the same time. You would have to have some serious circuits to handle the inrush as both pumps kicked on. The switch contacts on the little pressure switch wouldn't handle this load, so a motor starter would have to be incorporated into the system.
This might work, but it isn't a five minute project and could be more trouble than it is worth. Save your scrilla and buy an appropriately sized compressor and be done with it.
Stuart
