The pair will take to the stage to perform their new single 'Everything Has Changed' together at the live final on June 8.
Their appearance follows special performances from the likes of Jennifer Lopez and Demi Lovato during this week's live semi-finals. Olly Murs is performing on tonight's (May 31) live show.
Jack Carroll, Gabz, Arisxandra, Richard and Adam, Luminites and Pre-Skool have already secured their places in the June 8 final.
Four more finalists will be chosen from the next two semi-finals as well as a wildcard act, which is expected to be chosen from the losing third-place semi-finalists from across the week.
Britain's Got Talent continues tonight (May 31) at 7.30pm on ITV.
Photo gallery - Britain's Got Talent semi-final show 3: