Portal 2 texture overlay?

After taking another look at Portal 2's SDK, I noticed that the Hammer editor has identical, sterile tiled textures on the floors and walls, but in-game it has the Portal 2 Overgrown style to it, with some of them damaged or chipped and they are all different, the same with the walls, but they are all on one face, and I assume one texture, does anyone know how this was achieved, and how to replicate it in Unity?

 

A shader that combines two textures and two sets of texture coordinates?

 

imaginaryhuman is right. The simplest way to do this is to overlay a "detail" texture over top of the base texture. Unity comes packaged with one example of a similar effect using the Diffuse Detail shader.

You could overlay a grime texture on top of your normal wall panel to add variation to it, and make the tiling much less noticable... although for large, non-repeating detail, I'd either go the other way, and overlay the wall panel texture on a large-scale "overgrowth" texture, or write my own simple shader to take advantage of multiple UV channels.

 

While trying to use the Diffuse Detail texture, I looked around for a normal'd version, does anyone have a version of this with a normal map slot?