Although ideally batteries should be of the same age, type, and size category, adding a second battery will certainly give you more 12 volt DC reserves to depend on. Biggest issue is they will charge and discharge at different rates, if all you are looking to achieve is getting extended service between adequate solar recharging, you could expect an extra day or two of service.
To achieve the 100 percent success rate desire we use a multi-pronged solution when using our rig off the power grid. Since it is our preferred method of RVing our 120Vac and 12Vdc power requirements must be stable and reliable for indefinite periods of time. Of paramount importance when restoring consumed amphours via generator is the converter's ability to smart charge battery bank