The first challenge I was faced with was understand how to get the shaders on both sides. In order for this shader to work you would need to work with the "facing normals". This expression is what will do the trick.

if (switch == 1 && n == nf) //Float-Switch is always true//

else if (switch == 1 && n != nf)

Next to get the surfcolors to work just right you will need to give two surfcolors for each (one is true and one is false)...

surfcolor1 = texture(OutSideTxt);
surfcolor2 = texture(InSideTxt);

surfcolor1 = texture(InSideTxt);
surfcolor2 = texture(OutSideTxt);

The only difference between the two "if statements" is that they texture colors are switched.

I next put the surfcolors into a mix with a blend switch. This will allow me to blend and get a mix both textures.

Lastly, I wanted to add a multiplyer to the opacity so we could get a little transparency with the blended colors.

Here is the source code.....