Four ideas spring to mind: Idea #1: Try multiplying your two diffuse maps, instead of adding them. Idea #2: Try adding your two diffuse maps, and then subtracting .5 from the result. Idea #3: Try adding your two diffuse maps, and then dividing the result by two. Idea #4: Experiment with different blend modes in Photoshop.…
Sounds like your building is using a dark detail texture, and your terrain is using a light one. That's not gonna work. I recommend making all your detail textures with an average shade of mid-gray (127 in photoshop). Adjust levels in photoshop to ensure this. Once that's done, you can balance your shader correctly because…