On some of the maps I have merged in the past, I left room for a few map "blocks" between the maps if they weren't level to one another. This allowed me to create a mountain or something between the two maps and build a helix inside to allow the train to transition a huge amount of elevation in a small area. Google images of "model railroad helix" to better understand. You don't even need to disguise it if you have a good imagination or just ignore that area between the maps.