The heart cannot be used for two purposes
It means that one's mind can only focus on one thing at a time. From Xinlun zhuanxue.
The origin of Idioms
Liu Zhou of the Northern Qi Dynasty wrote in Xinlun zhuanxue: "if you make the left hand draw a square and the right hand draw a circle, you can achieve something for a while. Although you hold the heart of rules and return to your hand, if you can't, if you can't use both your heart and your hand, you can't use them at the same time."
Idiom usage
Used as an attributive or adverbial, etc
The heart cannot be used for two purposes