have one’s hand (/heart) on one’s halfpenny; one cannot keep one’s mind on two things at the same time; one should concentrate on one thing at a time; single-minded
❍ 自古道～。(凌明初《二刻拍案惊奇》卷六)The ancients put it well，“The mind cannot be devoted to two things at one time.”
心无二用
one cannot keep one’s mind on two things at the same time;one should concentrate one’s attention on one thing at a time
心无二用xīn wú èr yònɡ
形容专心致志地干某一件事情。one should concentrate on one’s work, one should concentrate one’s attention on one thing at a time, The mind cannot be devoted to two things at one time.