I hope I don't bore you guys to tears with this, but I felt the need to vent. There are quite a few youtube videos where people claim to have constructed a giant air compressor by simply connecting together the output lines of two small compressors. That is clearly not going to work correctly because these 'tubers don't seem to understand an air compressor's theory of operation. You can't use two regulators to control one compressor because one of the regulators is always going to shut off before the other one does, even if only slightly before, and then the compressor that's still running will start pumping to the atmosphere, either through the other compressor's intake valve or its unloader valve, and will keep running forever. Maybe you could correct this by using one-way valves, but it would be a kludge. Here's how I built a 2-compressor system (picture). I use one regulator for both compressors, and since I doubted its contacts could handle the combined current draw, I use it to control a Worldwide WDP60-3L-230 60-amp "definite purpose contactor" for the two motors. I mounted the contactor in a Square D NEMA box no. 9991UE1, which is about 6 X 3.5 inches, and which was supposed to be the correct size for the contactor, but if I had it to do over again I would have used a slightly larger box because the contactor just barely fit and it was too cramped making the connections. I'm not a compressor expert, so if anybody thinks I've made any mistakes in constructing my 2-compressor system, feel free to fire away.