Combining two regions is an absolute pain in the bum, however there are a couple of ways it can be done, though they are time consuming. The easier is to lay down the regions in strips laying down first one region as a lowlands and rivers base covering the entire area to be populated right up into the mountains. The second layer is the grasslands between lowlands and mountains themselves their height maxing where the mountain textures will begin. Lastly the mountains themselves again all of these regions overlapping well into each other. The last one to be laid, the mountains is then stretched to cover everything. Dont worry with a minimum height set for its textures it will only write mountain textures and items to a minimum height but it will drag up all of the identities of the other regions with it, working cell by cell, checking height limits and drawing content for the region suitable.