For some reason the terrain model module doesen't appear (Bentley PowerCivil for Baltics V8i (SELECTseries 2) - Version 08.11.07.494) in the Tasks and I couldn't find a way how to turn it on.
Terrain model in the Task was introduced beginning in SELECTseries 3. You won't find this option in SELECTseries 2.
For more information about the Road and Site design tools, visit the Road and Site design WIKI at:
http://communities.bentley.com/products/road___site_design/w/road_and_site_design__wiki
Is there any other way to combine two models into one?
Try the merge surface command.
https://communities.bentley.com/products/road___site_design/w/road_and_site_design__wiki/12248/combine-existing-and-proposed-surface