Carl, Bob, Forane and all. See if I can put some of this to rest by functionally describing the operation of system I designed, and which I regularly fly in IMC.
I have two 10" SV's, two back up batteries and two ADAHRS. Each SV has it's own, dedicated, not connected to anything else, back up battery which will power all functions in it's SV for an hour. The two SV's share one of the ADAHRS during normal operation. I can, manually select which ADAHRS the SV's will share, or allow the system to choose. OR, I can manually select which ADAHRS will power either of the SV's (and the other ADAHRS for the other SV, naturally). When self checks detect abnormal operation in either ADAHRS (which has never happened), the other (aka back up) assumes the lead.
Please, no speculation of my system operation if you don't have the details. (Only two others have that information.)
Guess if you don't have guts enough to fly with this system, perhaps model airplanes would be a better choice.