With the new lifepo batteries offering a higher nominal voltage, the solution of wiring all your avionic batteries in parallel with a high capacity low voltage drop schottky diodes in line with each batt makes a lot of sense. You will always be pulling from the best batterie(s) and you never have to do any switching....ever. You can find 18A diodes which have only a 0.4V forward drop at digikey. I've done this for over 10 years with lead acid batts and never had an issue even on long flights with a transponder running.