2 batteries with one receiver
Just plug the second battery (from the switch) into any open channel slot of the receiver. Hook each battery to it's own separate switch/charge receptacle so they can be isolated for charging. Both packs have to be the same voltage, don't mix 4.8v with 6v. Preferably both the same mAh rating but not mandatory.
This setup is usually used in 1/4 scale or bigger, not really feasible for smaller planes because of the added weight.