Her boyfriend Josh was two-timing her with his ex—a girl named Rachel Wade.
"to deceive, cheat, betray," 1924, perhaps from notion of "to have two at a time." An earlier reference (1922) in a Kentucky criminal case and involves a double-cross or betrayal, without a romance angle. Related: two-timing (adj.); two-timer.
To deceive and betray someone; esp, to betray one's proper sweetheart by consorting with someone else: Two-Timing Boy Wrecks Girl's Dream
[1924+; perhaps fr two at a time; perhaps fr making time with two at once]