[flybouy]

To John's point I think (dangerous proposition) the only way to charge 2 differing ampacity battery banks would be with a multi battery charger and an isolation circuit in between the battery banks.

Can you do this with a simple A/B manual battery switch? Yes you can IF and this is a big IF, you have an effective battery monitoring system to accurately monitor the state of charge AND commit to monitoring and then changing the switch to charge the other bank. This would be a lot of time committed if using solar to charge, providing you have enough excess solar capacity in the first place. To set it up automatically in my opinion would not be cost effective.

Everything I've ever read on the subject states to replace all the batteries in a storage bank with the same age and capacity. This is an oversimplified example but if you have a 4 cell flashlight you don't replace 1 battery at a time.