I think its possible if you use a Mix material. You can put your "Off" texture in Slot 1 and your "On" texture in Slot 2, then use the Mix amount to adjust. If you set the value to 0, itll show only the "Off" material. If you set it to 100, then it will show only the "On" material.
At least I'm pretty sure thats how it works, someone feel free to correct me if I'm wrong.