We can merge 2 separate toposurfaces into one surface.
We can rejoin split surfaces. The surfaces to be merged must overlap or share a common edge.
Click Massing & Site > tabModify> Site panel (Merge Surfaces).
How to do the same for Revit API ?
If any one can help me with some clue or code sample for Merge Surfaces which are sharing 1 common edge. Will be a big help.