Knight, 47, best known as Peter Brady, and Williams, 51, aka Greg Brady, have already shot the episode in which they play a gay couple who moves in next door to the Foreman family, the New York Daily News reported Wednesday.
The unlikely duo may even share a kiss during the April episode, the newspaper said.
During the entire five-year run of "The Brady Bunch," Greg never kissed a girl and the word "sex" wasn't even uttered until the final episode, the Daily News noted.
Williams and Knight have both been married twice in real life. Knight is currently engaged to "America's Next Top Model" winner Adrienne Curry.