Ella - I only get this effect if Wall1 is curved. Is this the case for you?
If so, this is a known problem, and occurs even if the roofline is not
modified - just having wall1 higher than wall2 will make the additional line
appear. This has been the case since ADT1. The same effect happens with
partial height Wall Modifiers on curved walls. In both cases, this is due
to the need to "re-facet" the exact arc of the wall above and below top of
the intersecting wall or modifier.
Note that you CAN use the Wall Merge option to eliminate the dividing line.
This will preserve the cleanup in plan, but can make it tricky to get
perfect cleanup, because the wall will not automatically extend or shorten
itself at the intersection.
Sorry for the slow reply. I had marked this for later followup, but got
sidetracked by other duties.
Bill Glennie
Autodesk Building Industry Division