Sounds like a ground problem unless you just crossed the wires up on accident. I would try groundeing one pair of the lights somewhere else to a good frame ground. Probably not good to ground that many things to one area, it would make sense that for the pair that should not get power (with one switch on only) the path of least resistant to ground might be back up and around the other set of lights and thus turning on all the lights since halogen bulbs are not polar specific.