The pair rehearsed Monday at a Hollywood soundstage to prepare for the program, which was taped Tuesday and will air Wednesday.
E! said there is no word on what they'll sing.
"The performance will showcase a more intimate side of Cyrus and feature surprise guests for an unexpected re-invention of the songs that have defined her career, including several from her No. 1 critically acclaimed album, 'Bangerz,'" MTV said last week in a news release. "Miley and her fans have always had an intimate and direct connection through her music, personality and no-holds-barred attitude and the 'Unplugged' stage will serve as the perfect platform for her to show viewers what inspires and ignites her music and her passions."