Seal, if you wanted two sets of lights to be controlled by one switch, it could be done but here is what you would have to do. Run both sets of wirings to the lights as you would if it was going to be two individual sets, that means proper grounding on both sets, two relays functioning independantly, etc. Then you'd have the two switches, take the two "switch" wires (the wires that go from the switch to the relay, should only be one wire per set) and splice them together. Essentially what this will do is make it so the switch when activated or deactived, will send the signal to two individual relays, which are still each still receiving full power and working independantly. Doing it in reverse (one wiring harness spliced out to feed four seperate wires with one relay, one ground, one swithc) would be a bad idea, and you would end up frying things.