Running a load off a solar panel connected to 2x 12V batteries.
There should be no need to switch the batteries around as you describe.

It seems to be widely recommended that, in a system like this, the two batteries be connected in parallel, and to both the charging source and the load.  When the solar panel is producing more current than the load needs, the excess current will charge the batteries.  When the solar panel is producing less current than the load requires, the extra current will come from the batteries.  The battery charge/discharge "switching" occurs automatically as the current supplied by the solar panels and the current required by the load vary.

Using the batteries this way reduces the number and depth-of-discharge of the charge/discharge cycles of the batteries and so should increase the battery life.