We had an interesting outcome the other day at work while wiring dual air compressor's after following the diagram supplied with the pumps. We were using the supplied relays and our own harnesses. If one relay was put into the socket, the compressor attached would turn on. If both relays were in at the same time only one pump would run. I did a quick google search and noticed something quickly about relay coils in parallel doing funky things but no explanation. On my way back to finish this up I had the quick thought to leave one relay socket as already wired and then put my 12V trigger wire onto the otther relay's 12V output which then has both my relays powered on at the same time without having the relay coils wired in parallel. As far as we could see there were no built in diodes in either the relay or the sockets. It was some weird mystery electrics. I am still though trying to understand what was happening.