Terrain Prism collision box problem

I was experimenting to see if I could get a diagonal line running across the terrain.
The idea behind this concept was to have one texture set (Brave) on one side and a different one (Mulan) on the other.

I found 3 objects in the terrain block set that are triangular in shape, which were perfect for my concept.

But then the collision problem nixed my idea.
When I rotate the triangular block to face in the other direction to align it with the one I had already placed, the collision box turned red and refused to allow me to place the second one.
Even though they are not occupying the same physical space, the collision box is!

Realistically, I doubt this problem can be solved, but if you can think of a way to solve it, all suggestions are welcome.

Please note that the placed objects must be terrain blocks, because the textures I am using are only available for terrain objects.