For a quick off the shelf solution:
1) buy a diode based "isolator block". That will route your solar charging current to the two banks of batteries plus avoid discharge between the two banks. Basically, a couple high current isolation diodes with a heatsink. Both anodes connected to the solar panel. One cathode connects to each battery. This diode arrangement will send the charging current from the solar panel to the battery with the lowest voltage (lowest charge). The other battery's higher voltage will cause it's diode to block it from discharging. If both batteries are charged, hopefully you already have a trickle charge circuit on the solar panel to prevent over charging the batteries.
2) buy a "marine battery selector switch". That will allow you to select between the two banks to your load. Basically a high current, single pole, double throw switch, which also allows "both on" and "both off". A double throw switch with high enough current would work. Preferably "break before make" type. Center to the load, each pole goes to one battery.
This is not an unusual arrangement on boats, but with the motor's alternator more typically supplying the charging current instead of a solar panel.
Check the battery datasheet for proper voltages that are exactly correct for your battery. Otherwise use safe values.
I have a commercial solar battery charger that by default overcharges the battery once per month to 15 volts to equalize the cells. Otherwise it charges it to 14 volts. These voltages can be changed from the user interface.
If you use too high battery voltage, it will cause gas formation in the cells. Traditional wet batteries will dry, gel batteries not so much as they contain chemistry to minimize it. But they will suffer and the end result is diminished battery life.
The most important thing for me has been to learn that it's not a good idea to overcharge the batteries. While it "feels" like you have more charge in your batteries when they say 15.0 volts than 13.5 volts, the difference is quite small in reality, as the last couple of percents of charge raise the voltage very quickly compared to the normal operating conditions.
Best Answer
80Ah means "80 ampere*hours" -- approximately, the battery can give 80 amperes * 1 hour, or 160 amperes * 0.5 hours, or 40 amperes * 2 hours.
1800W refers to maximum power -- if you draw more than that, the battery may overheat and fail. At 12 volt, this means 150 amperes. This means that you should not discharge battery faster than that. Incidentally, this means the battery can be safely discharged in about 40 minutes.