Pretty much Bob. I'd not really fear too much putting the switch to BOTH for charging. When going from battery A to battery B one has 2 options A-off-B or A-Both-B.
One should likely not go A-OFF-B, as this might present the same load dump situation which will blow the diodes in the alternator. I always fret about disconnecting my solar during daylight hours, and try not to. When i have, and expected my battery monitor should power off instantly, sometimes it stays on for a minute or two and freaks me out a bit.
The reason i say to go both and not move the switch entirely to the other battery, is that time is needed at ABSV to get the battery to 100%. The amps required to hold ABSV start tapering as soon is it is reached, and "wasting" available solar power, but it can take hours for it to actually max out the SG, which is the daily goal. Cutting ABSV short will cause the battery to be undercharged, and this desire not to waste a solar watt by this method will just contribute to premature battery demise.
But bringing a depleted battery into the solar electron mix will likely drop the voltage below ABSV and take X amount of time to get the depleted battery levelled off with the primary battery and then both can spend time at ABSV together. But likely it will not be for long enough to reach 100% and maximum Specific gravity, and the waste no solar electrons by this method can be detrimental to battery life as opposed to just wasting the extra power and holding the battery at ABSV for the time required. This can be 2 to 5 hours. Duration at ABSV depends on the battery, depends on the initial charging rate, depends on the depth of the previous cycle, depends on the temperature, depends on the health of the battery, depends on the Moon phase and possible presence of Locusts!!!!!
All in all, this desire to not waste any solar power can lead to issues or mistakes along with the extra cost involved by implementing methods to not waste the extra.
My flooded group 31 needs ~ 2 hours at 14.9ABSV and then a 15.3v finishing charge applied for a few hours more to return SG to near maximum. I pretty much need 16V to max out SG but this is hard on the battery, so 99.5% reached after a few hours at 14.9absv then a few hours at 15.3v finish is the compromise I have made with my voltage adjustable system when cycled nightly.
When the battery is not drained the night before, I do not need to give it the 14.9 or 15.3 for hours and hours, and I change the setpoints and durations way lower. But to get to near 100% cycle after deep cycle, the 14.9/15.3v hours long regimen is keeping my battery doing tarzan yells and thumping its chest like a meth addled gorilla.
Anything less after a deep cycle, and it becomes a drunk Capuchin Monkey afraid of its own shadow.
Depending on the state of charge of the secondary battery, it might force the primary battery into Capuchin Monkey territory.
I am NOT recommending 14.9 and 15.3 for every flooded battery. These values were determined by much observation, experiments and tools and desire and curiosity to measure the results of different voltage setpoints. If I could not adjust the voltage setpoints and used the 14.2v and 13.2v default setpoints of my solar controller, this battery would have failed completely 6 months ago.
Most all AB both switches are 'make before break', but there is break before make switches and these are not desirable for our purposes.
