Personally I prefer two suitably sized master switches that allow
either or both batteries to be on the main buss.

Darryl

Yep, I have flown with two switches for years. Simply switch on the
new battery just before switching off the old battery. Please, don't
tell me some horror story about "exploding" batteries! I have taken
readings with a 10v 'dead' battery connevted in parallel to a 12.8v
'fresh' battery and all I read was 300 m/a as the fresh battery tried
to charge the dead battery. And don't tell me about voltage spikes
either, because all those red-necks jumping their car batteries seem
to be surviving quite well, don't they?
JJ