I dont like adding complication to simple electronics myself I like the KISS method Keep It Simple Stupid.

although 5V relay circuit might work for whatever pack mah or voltage would have to sit down draw it out and find the relay (digikey is a good source), switching between batteries using a normally open contact on one pack and normally closed on the other. One pack would power the circuit closing the relay and sending its power to the switch and all the radio components when this battery failed or dropped too low the relay would open up and switch to the other pack.