The pair's respective characters Louis Canning and Patti Nyholm will team up against mutual adversary Alicia Florrick (Julianna Margulies) in a case that dominates season three's final episode.
According to TV Line, Patti will also be accompanied by a male nanny called Michael.
The character, described as a "stunning, model-handsome African man" with "ongoing concerns about diaper rash", has yet to be cast.
Fox was confirmed to be appearing in several episodes of The Good Wife's third season back in September 2011.
Plimpton has previously cameoed in three episodes of The Good Wife across its first two seasons.
The Good Wife was one of 18 CBS shows to be renewed for the 2012-13 season last week.
It currently airs on the network Sundays at 9/8c.