A couple suggestions you can try:
1. Split the Terrain in the text file used to import the survey. You can split it into multiple text files to import to separate Field Books and create separate terrains. This of course will only help if the data is easily split.
2. You can try to add in a Void over the area that is splitting the terrain data. Since there is a gap in the information in the two areas you want the terrains created from, the void should prevent the triangles and features of the mesh from forming in the void area.
3. Create the terrain by importing the Elements from one section into a new terrain. using the 'Create Terrain by Graphical Filter' or the 'Create From Elements' should work.
4. Export the terrain model to a file, reimport to a new terrain so you have duplicates. Then you can delete information not needed to split the terrains.
