This setup does not allow both batteries to be connected to the same
"bus" at the same time. On the rare time I've had to switch due to old
or insufficiently charged batteries, I've had no trouble with the
races from the loggers.

My question is, is this better, equal, or worse (from the standpoint of
battery usage/efficiency) than the more common "use one batt until it
dies, then switch" 2-battery setup.


I think you have a simple, efficient method of using your batteries.

The only time having both batteries on at the same time is an advantage is
when they are both so depleted that neither one alone will operate the
equipment, or the current draw is so great that the capacity will increase
by dividing the work. Your system has the advantage that it works and you
have specific information from the work performed by each battery.

There is a slight potential problem with having two batteries in parallel.
That is if one has a shorted cell. In that case, the good battery will try
to charge the bad battery, wasting energy. That could be solved by the
diodes, but they waste energy, also.

There is a great deal of merit to keeping something as simple as possible
and remembering that if it works, don't fix it.

Colin