Dual battery solar controller, What are you using to charge both batteries from your solar panel.
Two group 31 deep cycle Deka batteries in my truck,
one starting and the other as a back up .
I keep my float voltage at 14.5 volts all the time, so stage charging is not something that I monitor, I only notice that a battery connected to the load out behaves like the one connected to the battery out. I don't notice any difference between charging from battery out or load out. It seems they might be both on the same circuit. I did notice if I have a battery on the load out, I can leave the battery out with no battery and controller will still operate.
The only big difference being the load out has an overvoltage relay that will cut off power if the voltage goes above a certain setpoint to protect 12 volt devices connected to it. if this relay is triggered you have to manually turn the load out back on.