Hello sunalp!
...merging tracks... hmm...
In all the examples illustrated above, You will get as a result "double faces" on the join points of the two tracks (flickering, rain reflection problems and so on...)
The extra terrain mentioned by sunalp2 idea works well, but with obvious mapping problems.
As a conclusion, in both cases You will have to use another software to accomplish the job:
Two merged tracks:
Blender or 3DSMax will solve the problem by eliminating the unneeded faces and joining the track edges
Extra terrain solution:
Blender or 3DSMax will solve the problem by mapping the area of interest
Freedom of choice :)
Here 3 pictures that illustrates the overlapping problem/solution
Ivora - Cambridge International Centar Hrvatska