There are so many variables that come into play that I don't think this plan would work. Primarily you will have to deal with the regular upgrade list. if the passenger in the "C" seat is not already on the list they would find themselves at the bottom. There are countless other scenarios that could thwart your plan.

In a perfect world with an empty flight this could work, but if you have many SWUs to use, why not use 2 and upgrade the both of you?!