Recently I have been browsing power buses on the net whilst having nothing to do or can't be bothered to do anything. So this may open a "can of worms" or a decent debate. On taking apart a few old receivers I have found that the power pins, + & - are connected to the same copper strip/bus. The receiver draws it's power from the bus.
The signal pin from the receiver circuitry. The RX voltage is probably 3.3v or 5v and the current minimal. Therefore the current handling capabilities of the receiver are purely down to the internal copper strip or common bus.
This got me thinking why off the shelf power buses (without integrated receivers or gyros) are fairly expensive to outright astronomical for a simple bit of circuitry. My scribbled drawing show how simple it is. Between the two battery inputs you could put in a voltage comparator with a few extra bits would switch each pack according to their voltage. Or you could just use a shotsky diode to isolate them. There may a bit of a voltage drop across the diode but if using high volt servos from 2 cell Lipos would not be critical. In fact some "so called power buses" for two packs are internally connected together so does not really give you battery redundancy.
I am thinking of making my own 9 channel when the rains return......
Apart from my Decathlon which has a twin powerbox switch - All my larger models have successfully used two batteries via shotky diodes and switches - One to the battery connector. The other via a y lead to the last channel on the Rx (which equalises the current on the RX bust these models do not uses high power digital servos.