First of all: make sure 3D view is set to textured.

Now if you look closely at the Skin EDitor you'll see 4 triangles.

The two on the left that form one rectangle are the frontmapping and the two triangles at the right form the back mapping.

If you want to have the yellow square mapped on both sides you have to select the frontmapping and stretch it, then select the backmapping and stretch it too. Now if you do it correctly you'll end up with a X shape. And the cube will be mapped correctly.