Even if I use Schottky diodes, I think that I'd be wasting ~1W (0.5V forward voltage times a couple of Amps) out of a total power budget of ~25W or less.  I did think about diodes, but I'm trying to be very energy efficient here.

And you're probably right that I could avoid DP relays, but I get a warm feeling inside totally isolating the unwanted supply, and the DPDT relays didn't seem (more) expensive...  B^>

But, yes, thanks for the observations: they're spot on!

Rgds