I vote for simplicity - a fuse at each battery, a panel switch for
each battery, a fuse for each instrument, a switch for each
instrument, and a voltmeter (discrete or in an instrument). If its a
long flight for a badge or record, use a separate flight data logger
powered from an independent battery, and turn off instruments you
don't need. No circuit breakers, no diodes, no FETs - save that energy
for the avionics.

Fly with one battery until its voltage goes below 11.0, then switch to
the other battery. Switch the battery you fly with first the beginning
of next flight. You'll then know the performance ability of each
battery during the season, because you'll know how long it supplied
your exact flight needs, at what temperature, at most one flight ago.
And of course, charge the overnight batteries after the flight.

Automation is fine, but battery management is so simple why waste
electrical energy? I bet there isn't a glider pilot out there who
doesn't look at his battery voltage at least several times during each
flight, and who doesn't understand switches...

-John