Nyal Williams wrote:
Seems to me that in order to take care of the Volkslogger,
the ideal switch would be a 3-positionswitch: 1.
Battery A; 2. Battery A&B in parallel; 3. Battery B.
Switching rapidly from 1. through 2. to 3. would prevent
interruption in power without raising voltage or otherwise
causing damage, and position 2. could be used when
and if both batteries are somewhat low.

Does such a switch exist that provides such an overlap?

Sure it's called "make before break" some one already
has mentioned it.
...lew...