According to E! News, the Material Girl and the "Wrecking Ball" singer were seen on a Hollywood soundstage Monday rehearsing the collaboration.
The stripped down acoustic concert will be taped in Los Angeles Tuesday and aired on MTV Wednesday night. Sources told Us Weekly, the duet came together "last minute."
There's still no word on which song the blondes will be singing, but according to MTV, the concert will feature "more intimate reinventions of her Bangerz tracks."
Cyrus -- who was named MTV's Entertainer of 2013 -- joins artists like Nirvana, Jay-Z, Katy Perry, Lil Wayne, Mariah Carey, Eric Clapton and Adele, who have gone Unplugged since the show started in 1989.