Metal Ground was running a series of videos: Pop Singers VS Metal Singers. They may still be running the series... A clip from this performance was used to highlight hymns. Christina Aguilera performed "Ave Maria," and then there was this duo.
Please note: There is no AutoTune anywhere to be found on this song.
"Stabat Mater" was originally a Latin hymn from the 13th Century. It has been arranged several times in the intervening years for chorus and orchestra. This arrangement is credited to Giovanni Battista Pergolesi. A version was originally included on the 2013 Epica album Retrospect.
This is "Stabat Mater Dolorosa" performed live by Simone Simons (of Epica) and Floor Jansen (of Nightwish). Though the song is usually just listed as "Stabat Mater," this is how Simone Simons and Epica listed it for this concert.
I may revisit some of those other match-ups eventually, because they are worth a look.