I'm probably way over-simplifying this, and likely aren't comprehending your original question, but it sounds like you have two existing morphs on the same object and wish to create a master dial that controls both.  If this is correct, then why not just "Create new Master Parameter" on the figure?  I taught myself how to use it in about 5 minutes without touching the manual so that's gotta tell ya how easy it is.  I've managed to tie dozens of morphs to the same dial.

If that's not what you were trying to do, then just ignore this.   :)