Here is one possible solution:
1. Use a Jeti standalone BEC (several units to chose from), connected to your flight battery. Cut off the switch so it is always ON (or leave the switch and just hold it in the ON position). The standalone BEC will be connected to your flight battery (assume 10s)
2. Use a Jeti ESC with built in BEC (not the OPTO version). Cut off the switch so it is always ON (or lave the switch and just hold it in the ON position). The ESC will be connected to your flight battery (again, assuming 10s since this is F3A)
3. Decide which unit you want to be PRIMARY and which unit you want to be BACKUP. Set the PRIMARY to say 6.0VDC and the BACKUP to a lower voltage, say 5.8VDC.
4. Take the standalone BEC output AND the ESC/BEC output and connect them to the inputs of a Jeti DSM-ESC.
5. Connect the DSM-ESC output to your receiver.

The DSM will automatically connect the higher voltage to your receiver. In normal operation, this will be the PRIMARY. Should the PRIMARY fail, then the BACKUP automatically is connected. If you have telemetry on your radio, you can set an Rx voltage alert so you will know the switchover has taken place ; otherwise you would have no idea just flying the plane.

No switches; the system automatically powers up as soon as your 10s flight battery is connected. You will need a simple Y adapter to connect the flight battery to both the ESC and BEC.

There are many other configurations that would address your concern, but this one is pretty straightforward and simple.