two-time

From King Dictionary of Contemporary Englishtwo-timeˈtwo-time verb [transitive] informalRELATIONSHIP to have a secret relationship with someone who is not your regular partner SYN cheat He doesn’t know Claire’s been two-timing him.two-timer noun [countable]→ See Verb table
Examples from the Corpus
two-timeIt turns out Ryan was two-timing Jeannie with her best friend.