Simple Bus-Tie Circuit using Schottky Diodes
When using a separate Universal Battery Elimination Circuit (UBEC) to power your receiver and servos, the Electronic Speed Control (ESC) integrated BEC isnít utilised. This simple bus-tie circuit allows you to connect the UBEC and the ESCís integrated BEC so that in the event of the UBEC output voltage dropping below that of the ESCís BEC, the ESCís BEC will instantaneously take over. This provides a little extra peace of mind for little cost and only a few grams in weight.
Here is a link to the full thread.
This shows you whatís required, how to put it together, how it works and of course, some discussion.