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."
Swim Week Miami Beach 2014 [PHOTOS]