Charging a single 12V battery from 2 solar panels
I assume you are mentioning something like a box with solar panel on either sides.

Consider below image.


The wall adapter and the 9V battery can be your two solar panels. We call them Panel A and Panel B.

When Panel A has more sunlight incident on it, Panel A voltage will be higher and hence Panel A will conduct and it will supply the load.. The diode in front of Panel B blocks the current entering it and hence, practically you can treat Panel B as like not connected only.

Same thing happens when Panel B has got more light energy. 

The diodes should ideally be very low leakage ones(BAV720, for example) which helps in blocking leakage and battery discharge through the panels when there is no light.

Low leakage diodes normally come with little higher forward voltage drop and hence you have to decide based on applications and voltage margin you have. 

Edit 1:

If battery power is not to be truly saved for long, I would suggest then to go for Schottky diodes or even PMOSFET based polarity protection. Here, almost all the voltage is available for the battery and loss during charging the batteries due to voltage drop of the diode.

One point here: you are completely losing the energy harvested (no matter how small it is) from the panel which is upside down.

Edit 2
The load in the picture shall be treated a charge controller module for batteries