Steve Paavola wrote:

If both batteries are on-line all the time, how do you know when one is
getting weak and needs to be replaced? Or do you replace both batteries
when voltage is marginal at the end of a flight?

I've been using the 2 toggle switch solution for years. I replace a
battery when it discharges within 5 hours. This means I've always got at
least 10 hours worth of battery (assuming I've recharged the batteries).

It doesn't really matter, when one goes bad, you should replace both.
Not only will a new battery not load share properly with an old one, if
one battery in the set failed, the other can't be far behind.

The same thing goes for your two bulb light fixture at home. If you
only change one bulb at a time, you will just be changing bulbs twice
as often.