One Mesh, Mutliple FileTexture couldn't?

I am trying to cover two FileTexture(Front Image and Back Image) in one box mesh.
But I was not able to find a way.

this thread looks like similar question for me.
but @joreg’s answer, I can’t understand that. ㅠㅠ

anyway, what I want is simple.
One mesh object - Front FileTexture, Back FileTexture.(I want to use two ImageFile.)

I attached my testing patch.
could you comment about my question? (1.4 MB)

try like this

boxchanger 1.rar (1.4 MB)

Thank you!! @antokhio!!!
Cons(EX9.Texture) Texture Count 3 looks a key.

actually it’s tweaked x model itself