The two battery pack idea seems like it would work, but there are somes things to do first.
a) you should have a blocking diode, Schottkey preferred as it will only drop
about .2 or .3 volts versus .7 volts for a silicon diode, in series with the
positive wires going to the receiver. Because if you just plug them in to the receiver
in essentially parralel configuration, If one pack goes low, the other pack will try and charge it up.
They will sacrifice powering your receiver for being balanced electrically.
If however you have a blocking diode, the one pack won't see the other.
Put a secondary charge jack on the battery side of the diode if you use a charger that needs to sense
battery voltage, if using a wall-wart, don't bother with the added charge jack, just charge each pack
seperately