Hooking up 2 batteries is a pretty easy thing to do. This is normally done to get redundancy in batteries in case there is a failure. If this is what you are going for then it's also best to duplicate everything in the battery setup including the switch. This would mean two batteries and two switches.

To accomplish this you would setup each battery on it's own switch and then plug each battery into a separate channel in the receiver. If you don't have 2 open channels on the receiver you can use a Y-harness and plug the harness into a channel of the receiver and then plug the servo for that channel into one side of the harness and the battery into the other side of the harness.

By doing this setup if you have a battery or switch failure the other battery and switch will still provide power to the receiver.

Ken