Are you talking about piecing together parts of two different textures and making the two match? I haven't dealt with precisely that problem, but I have done something similar when compositing photos that should match in color, scale, etc. What I do is make sure the edge of the image I'm matching to the main image overlaps substantially. Then I go into each of the color channels, usually red, green, and blue, and do whatever dodging and burning is necessary to make a smooth match on that channel. When the channels all match, the composite colors will match. Finally, I create a layer mask on the image I'm matching and use an airbrush tool at low opacity, about 5 to 10%, to feather the edge of the image so the transition from one image to the other is indistinguishable. I learned to do this because I needed to digitally composite aerial photographs. It works, I've done a lot of it.