Revit automatically creates joins between intersecting walls. You can join the geometry of closely-spaced parallel walls.
 Geometry panel
Geometry panel Join drop-down
Join drop-down 
 Join Geometry.
 Join Geometry. 
If either wall has an insert (such as a window), it cuts through the joined wall. Any geometry around the insert, such as a frame, does not display on the joined wall.

Parallel walls being joined