Can anyone tell me where to find a diagram on hooking two large shop air compressors together?

It does not matter how you hook them up as long as they are both on the same air line. I assume you are trying to increase the capacity. The regulators on both should be set to same psi. They both will come on when pressure is low. I hope this helps.

I would run a pipe from the outlet of one to the outlet of the other, then draw from this crossover to whatever you are operating. Be sure that the kick-on and kick-off pressures are set the same on both compressors.

Both of the previous answers seem reasonable..but..you may need to make sure they have the same size outlet...one may have a larger pipe and move more air. ie. one compressor tank may empty faster than the other. or one may fill back up faster than the other.

Plumb both outlets into a common tank. Both outlets must have Non-return valves fitted.