In my opinion you want to be able to turn on the backup to be able to check to see if it's working. I have hundreds of flights using the Jeti BEC driving high voltage servos. My backup system is a Tech Aero PLR5 set at 6.2 volts with a 350 ma 2 cell lipo.

http://www.tech-aero.net/plr5

I check both sides every flight. I turn on the backup regulator first. When the servos kick in, I turn it off and plug in the main flight pack. When the BEC kicks in the servos, I turn the backup regulator back on.