Jim,
If you use a single pack your system is drawing ALL of it's power through the two wire lead that comes off of that pack and delivers the power to the receiver bus. Adding a second pack adds another set of leads that feed the bus and thus each lead, from each pack respectively, only sees half the amp draw, reducing the possibility of a voltage sag. Of course this assumes the packs are of similar capacity and voltage.

Spias,
There is nothing special about li-ions. The 2-cell flight packs are nominal rated at 7.4 volts per pack which is too high for most radio equipment. This necessitates the use of a regulator to knock the voltage down to around 5.5 to keep the gear happy. All you need to use them would be a regulator per pack and a switch per pack, assuming you run redundant packs. There are 4 sell packs but they are nothing more than two 2 cell packs in the same warpper that are wired in parallel, you get higher capacity but the same voltage.