fall between two stools idiom
两头落空
If something falls between two stools, it fails to achieve either of two aims.
The grammar guide falls between two stools - it's too difficult for a beginner but not detailed enough for an advanced student. 这本语法指南对初学者和水平高的学生都不适用——对初学者太难，对水平高的学生又不够详细。