Hi, Use caution when using two batteries with two switch harness plugged into the same receiver and turned on. This set up will only protect a model in the event of a battery failure caused buy an open cell or a broken solder joint seperating the cell in the pack creating an open circuit. if one battery shorts or discharges then since the second battery is connected to the battery circuit in the receiver then it will run down the back up battery. The only real way to correctly use two battery pack is by using a battery back up system that electronicly turns off the problem battery and ingages the back up battery. There is one available from EMS that works very well and has an LED thet turns on when/if the battery goes to the back up battery so you will know th check things out. There may be other systems out there but this is the only one I know of and they work.