Answer: Deloris and Mary Robert
After Vince, Willy and Joey are arrested, Deloris and the nuns make it back to the church to perform for the Pope. The choir starts singing 'I Will Follow Him' together, then after Sister Alma jams out on the piano, Deloris starts singing her solo. As soon as Deloris' solo is over, Mary Patrick and Mary Robert sing a mini duet together. When they finish it, Mary Patrick backs away and files back in with the other nuns so Mary Robert can sing her solo. This part always give me goosebumps!