I have an old Cub with one battery and no solenoids, battery positive feeds dir to a dual 30 amp fuse block than each fuse dir to a DPDT battery master toggle, from the factory both throws fed the main bus (individually) gave you a second 30 amp fuse to power the bus if one blew. Dirt simple and never had a problem, thousands still use this original setup including me. Only time I had a 30 amp fuse pop was caused by my own accident grounding of that DPDT toggle while working on generator to Denso alternator conversion. The other pole of the battery master toggle has a jumper connecting both throws and was Gen field power, when I installed the Denso 10 years ago I cut the jumper so I could use one throw Battery only without the Alternator. A good setup, no solenoids to go bad and no issues over thousands of hours and thousands a planes still flying this way. Has no starter solenoid either, factory setup was starter is hot off the same imput to the battery side of the dual 30 amp fuse block, has a manual BIG push button starter switch with Heavy Duty contacts, like a manual finger powered start solenoid, works fine and never had an issue, the one thing you have to be careful of is the starter is ALWAYS hot, factory setup and thousands still flying today like this. The starter button is BIG and mounted under the seat from the factory, they call it the Piper anti theft switch. Works great.
So what you could do is have both batteries feeding a dual fuse block like the Piper setup except you would have a separate battery for each fuse instead of a single battery feeding both 30 amp fuses. Alternate between each battery every flight, this would give you a backup battery so you may never have a dead battery problem ever again. You would have to have two starter switches though in order to keep the batteries isolated, or a switch that breaks the connection after starting. Two good AGM batts together would really put serious AMPS to the starter though, make sure you have good brakes