Applying for different PhD positions in the same research group
It is okay to apply to both positions.  The student might even be at an advantage, having interests and skills in both areas.  I might try to contact both professors in advance, and ask if they would like the application structured as one or two applications.  If the student does submit two applications, I would recommend stating in the cover letter that they are applying to both, and would be excited to work in either position.