Mapping multiple image textures of different resolution and extent to the same mesh

I’m using a terrain model with a UV texture of aerial photos overlaid on it. The thing is, I want to zoom in on the terrain and use a higher resolution texture on the part where I zoom in. I can’t use a high resolution texture on the whole thing as it would be way too big. However I can’t seem to overlay one UV texture on another, it either shows one or the other. Is there any way to solve this?

In the image you see a UV texture with clip enable in the texture mapping. Where it is clipped I want another UV texture to show.