I think I'm being paranoid a bit but want to double check something. Recently upgraded to a new Battery Bank and new panels. The old bank still has life left and I have a dual CC from the old panels charging the new bank and the old (while a separate charger from the new panels is charging the new bank).Everything is working great but the old bank is not doing anything. I don't want to run it in parallel with the new bank but would like to be able to switch the DC distribution to it as a backup. I should note the system is not large and the inverter is rarely turned on.

First jpg shows a very simplistic diagram of the current setup. All I want to do is insert a battery select switch to switch over the DC distribution to the old batteries. This does mean that the negative side of things is not isolated between the two options (in a perfect world I would find a battery switch that handled both the Pos and Neg (DPxT kind of thing). Second jpg shows the change. This would allow me to, if needed, run something heavy of the inverter for longer but switch over the DC for lights etc until the main bank could be recharged.

I cannot see any issues with the shared negative (and this is certainly how trailers etc would do things) nor can I see how it would confuse either of the CCs. Am I missing anything?