I don't like GIMP, so I used Photoshop. However, GIMP should have the same thing...
Made 2 layers that are the same dim as the two patterns collaged, but one layer the 'sand' one and one the 'cave' one. Now, on the top layer, go Layer -> Apply Layer Mask. Apply a gradient so that it goes from pattern to alpha. Presto!
Edit. Pretty much that Texture Merger above does the same thing, but you have to go through applying the alpha to black gradient first, which is synonymous to the layer mask, and then the Texture Merger program just does...what layer mask does... >.>