Hello: I’d like to see a system diagram for the two tank system you’re thinking about. It would help clarify things for me. In general, I prefer simple as there is less to go wrong and need to fiddle with. Also, how fast can the geothermal recover, say heating up a tank from cold?
Checking out your current tank should start with looking at the anode. It will tell you all about the condition of the inside of the tank 😉