As a matter of fact I've seen a lot good post on these thread, there are plenty of ways on doing these to avoid power failure on our flight control system, although we can still maximize its advantage but the most important thing is the correct circuit and rating. I found our Rx power bus has thicker soldered lead to provide higher ampacity and lesser resistance but I still practice to leave one to the battery port and others in between the load slightly farther from original port. Although it may not matter due to my servo normal requirements but connecting one or more power and load to any port contributes power network to my tiny grid. I also use redundancy on my main battery cable each can provide 5A. I used regulator that maintains 6.0V all the time and monitor battery and Rx voltage using 2-channel display as you can see on the photo. The backup power is standby and can take over whenever voltage at Rx drops to 5.7V.
Direct connection of 2 identical A123 battery to the Rx using deans connector is the simplest I can see but using HD power switch makes it more convenient. While providing regulator controlled by fail-safe switch has another advantage of technology since most of the regulator of today are not just regulator but provides redundancy.