The double-elimination competition Tuesday drew boos from the ballroom audience.
Lachey and Castroneves and their professional partners, Anna Trebunskaya and Chelsie Hightower, were joined in the bottom three by singer/actress Sabrina Bryan and partner Louis Van Amstel. Bryan had the second-highest score Monday night with a 25.5 out of a possible 30.
Lachey, who won Season 2, was given a 24 for his cha-cha, while Castroneves, the Season 5 winner, received a 25.5 for his quickstep.
Celebrities Emmitt Smith, Apolo Ohno, Gilles Marini, Kelly Monaco, Kirstie Alley, Melissa Rycroft, and Shawn Johnson also remain in the competition.