Sometimes one needs to cheat so to speak. I once combined 3 different renders to make a full canyon. Rendered the left side canyon with the displacements coming in from the left. Then rendered the right side with the displacements coming in from the right. Rendered the canyon floor with mild displacements then blended them together in Photoshop. Wasn't that great but it worked since I didn't know how to control things and still don't really. LOL