I approached dual batteries (both AGM) in my 100 a bit different using a simple 500a solenoid as an isolator.
The isolator is triggered by a two pole (on-off-on) switch inside. On one pole the relay is triggered via 'ignition on'. This is pretty much a set it and leave it setting. The batteries are separated when the vehicle is off. The 2nd pole is connected to the 2nd battery so that I am able to manually activate the solenoid to self jump start. The self jump start worked great one day in the shop when I managed to drain the starting battery. Not only is the system simple and dumb, it would be very easy to repair it in the field if necessary. It's been working for the past 3-4 years.
A couple of years ago I also added 100w of solar to the roof and use a basic solar charge controller to help keep the 2nd battery topped off when parked.
is one a deep cycle agm and the other for starting?