How do I apply Multiple textures to a surface?

For example, the walls in the sewer demo. They have the "leaks" texture, and then some other concrete-ish texture underneath that. But the inspector only shows the leaks texture for some reason. I'm going for a similar effect. I imported the leaks texture into my project, and I'm try to place it over the concrete texture I made in PSE. but it just replaces the texture.

"leaker" uses a shader that only multiplies in color. The main material for "concreteWalls" is called "waterfall-lambert2". You can either use the ones from that project, if you want the same effect, or write your own shaders.