Hi
@BBBFLASH
With two different batteries (100ah and a 120ah) you should really keep them separate.
Connected together the Setec charger will operate on an average voltage and not give optimal charge to both. They will also try to 'equalise' leaving one battery less than 'full' as the higher voltage battery attempts to charge the flatter one. The same reason you don't mix old batteries with new in a torch.
I have a setup with the original Jayco supplied battery and another different make 100ah that I separate really easily with
one of these. A couple of battery leads from supercheapauto and it was all wired up in under an hour.
You can switch it to connect both in parallel or simply call one battery 1 the other battery 2. I run batteries separately and in reality have one in reserve should the other run down to the point where low voltage cut out kicks in and switches the system off. (Just reach under the bed and switch to the other battery).
I tend to alternate batteries to keep everything fully charged. In reality I'm yet to run one down to the point of cut out so am even starting to consider if two are necessary for my mainly led lighting load. That said, a few cloudy days where the solar didn't charge too much and I'll be glad to have my reserve in place.
Hope this helps.