How to determine which texture was hit within material

Hello,

I have a wall material that uses two textures - an inside and outside wall. When I raycast and hit the wall, I need to know what texture was hit so I can place the right decal in the hit position. As you can see from the image below, I want to place a brick decal if the brick texture is hit and a concrete decal if the concrete texture is hit.

Is this possible using one material like this? Or am I thinking about this wrong?

 

It just occurred to me that since the material is made from a single texture, this might not be possible. Can anyone explain the proper way to accomplish what I need to do?