Most people using two batteries use two switches as well. Just plug the output of the second switch into any unused servo slot on the receiver. If no spare slots, Y into one of the other channels. The reason for two switches is for more reliability and being able to charge each battery separately (most switch assemblies also have the charge jack or lead). If not two switches you would have to remove the battery(s) from the fuse to charge as you can not properly and reliably charge either NiMh nor NiCad in parallel. This also lets you check each battery between flights, just check with first one switch off, then the second switch off. Both conditions should give you proper operations. Now switch both on and fly