She realized her boyfriend was two-timing her when she spotted him on a date with another girl.

two-timing 是由 two（二）和 time（时间）组成的复合动词，字面意思是"在两个时间段做不同的事"。 在感情关系中，two-timing 特指"同时与两个人交往"，即"脚踏两条船"的行为。