Two seperate packs!!
For sure two Batteries!! If there is a simple reliable way for the servos to have their own battery pack that maybe is the way to go!
I have experience with two batteries plugged into the receiver. One in the normal port the second in a Y connector any other port / channel. I use no regulators or battery backup devices, just two powerful high ma packs! Two separate switches one for each. Keep it simple but redundant ------
Lee H. DeMary
AMA 36099