How do I rewire my existing garage lighting so that one fixture is controlled by a new switch?
Exactly what you said. Disconnect the first light from the second. Run wire from the second light back to the location of the switch. If it is a single gang box, you will have to remove it and add a double gang box or put a double switch meant for a single gang box in. Tie into the power for the first light and use it to power the new wire to the second light.