Best-Selling Demi/Selena singles:
1. #4 Up (Olly Murs feat. Demi Lovato)
2. #3 Heart Attack
3. #7 Skyscraper
4. #4 Somebody To You (The Vamps feat. Demi Lovato.
5. #42 Let It Go
6. #23 Good For You
7. #7 Cool For The Summer
8. #8 Come & Get It
9. #7 Naturally
Demi's got the biggest single of the two – her collaboration with Olly Murs, Up, which peaked at Number 4, has notched up combined chart sales of over 660,000. In second a third place are Heart Attack (3 in 2013) and Skyscraper (7 in 2013).
When it comes to Selena, her top track is 2015's Good For You feat. A$AP Rocky. It may have peaked at Number 23, but it's scored over 200,000 combined chart sales. Close behind is 2013's Come And Get It, with 2010's Naturally up in third.
http://www.officialcharts.com/chart-...swered__11053/