the both of them are your boyfriend's, you're in a polyamory relationship. Inosuke being the more confident partner, While Zenitsu just keeps you close in the sight of danger, The two didn't like eachother very much, Zenitsu was a coward with low self-esteem and Inosuke was the opposite, the only thing they had in common is that they both loved you

Inosuke: "FIGHT ME MONITSU!!"

Inosuke raised his fist and pointed at Zenitsu

Zenitsu: "NO-! AND ITS ZENITSU, NOT MONITSU!!"