I really appreciate the inputs. Especially Zeeb, "not a good idea" got me to consider what if 12120 wants batt 2 but PBswitch had not switched. Now the Power Box switch only makes sense to me if it outputted to only one line.
I am still on 6v servos, 8711 NOT HV.

I not going to play chicken with a switch and a receiver than can each select batteries. And it looks like the 12120 can at times draw off both. I may set up an intentional test.

I have ordered 2 Life packs and will run 6.6 directly into the 12120. With those I can still mount them where I had the Lipo s and similar weight. And since I have open access to the power lines I dont even need the fail on spektrum switch, just pull the power leads. And I will see if it actually switches 2nd batt after every flight.

(May be a big lesson in just because a thing, PB switch, came with the airplane it may not be a good idea.)
Thanks again, may have saved a smoking hole in the ground
Charles