Here’s one way: Copy the line work from one file, open the file for the adjacent tile and use the command “paste to original coordinates” to have them together in one DWG. Use the command PEDIT (polyline edit). Select M (multiple), and choose all the linework from both tiles and hit enter. You will be prompted with, “Convert Lines and Arcs to polylines?” choose Y (yes), then enter the option J (join), the join type is “extend”. Enter a small fuzz distance 5 (mm). The line work from adjacent tiles with identical elevation values will be joined into single line features.