I know that the Aeroelectric method is to use a diode but that requires the secondary wire to the E-buss to be permanently connected. For some reason, I just don't like that idea. It could be switched but then there would be a delay with no power momentarily. My idea would be the relay with an ON-AUTO-OFF switch to bypass the relay if needed. I know it would work well - it's just a case of finding a relay. They exist in the commercial world but I need to find one at experimental prices!
To clarify, with power in the switching circuit (in this case, the main buss) the contacts on the power circuit (E-buss standby wire) are held open. If power fails (ie loss of main buss) the contacts close and the E-buss is powered by its' secondary route.
If I knew how to do it, I could post a diagram!