From Wiktionary, the free dictionary
From co- + favorite.
cofavorite (plural cofavorites)
- A joint favorite to win a sporting event.
2008 January 25, Larry Dorman, “Matteson Gets His Bearings and Grabs the Lead”, in New York Times[1]:Phil Mickelson, who was the pretournament cofavorite with Woods, struggled to a two-under-par 70 on the North.