two-time (someone)
to have a sexual or romantic relationship with two people at the same time.
If I ever found out she was two-timing me, I'd kill her.
英语词组固定搭配大全包含18674条英汉双解词组，基本涵盖了全部常用英文词组、短语的翻译及用法，是英语学习的有利工具。