Double belt, double champion (or simultaneous champion in cases where the belts are won without abandoning or losing the other) is an achievement made by mixed martial artists in several categories.
Some sports experts argue that a double championship could be detrimental to the divisions that the champion competes in. The progression of the divisions may be blocked, which can be considered unfair to other challengers. This can be due to the fact that MMA athletes cannot compete in short periods of time (except for rare circumstances, such as fights that end quickly and/or without injury) and must have adequate time for recovery and training camp. Furthermore, athletes need time to adjust their weight, which can further delay title fights.
Max Holloway of UFC, who won the featherweight and BMF titles, is not considered as a double champion, as the BMF title is not an official "weight class title". As well as Charles Oliveira from the same promotion, who won the lightweight and BMF titles, is also not considered as a double champion for same reason.