Thanks for all the help trying to get this to work...I didn't end up getting this to work no matter how I tried to set it up. I even tried a basic set up. As long as I used multi select boxes as the initial selection, it would never work. I suspect not many of the users have tried it this way as it is a bit uncommon to have a many to many multi select dropdown vs a one to many from a single dropdown as the starting point, so I am chalking this up as not possible currently.
Just letting everyone know in case someone tries this in their environment without beating their heads in wondering if its possible as I did. I ended up purchasing a 3rd party SP add on for a couple hundred bucks that does this out of the box. Setup took a couple minutes and it did what I wanted. I ran out of time on my deployment so this worked out perfectly.
Thanks again for the help, I wish it would have worked as this all the SPServices are great tools!
