merging or combining two walls

I am trying to create a floor plan for our offices.  There are cubicles to be designed.  I have drawn two of them and wanted to combine or merge (?) the shared wall.  When I try join it does not work, not sure what to do.  See the attachment, there are two images (join is what I am trying to do, join2 is a clip from the design I am working from.

Thanks in advance
join2.jpg
join.jpg