Yes you can run a single wire from your battery to the head units, amps, and any other equipment you might have with out an issue, mind you use an AWG that is appropriate for the power draw if it's just head-units 12-18 AWG is ok. But keep in mind that the shorter the ground wire is the better. Your best bet is to make a ground spot on the car's chassis very near by the head units, under a foot is best.

As for grounding a multi head unit install in different places on the cars chassis never never never you WILL get ground loops and if there's a problem now you'll have to check more wires to see what the problem is.