Increase the capacity of a 12v battery bank by adding new batteries
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.