depending on his personal preference and capability

Honestly, except for certain and very specific careers or positions, having both degrees does not do much other than looking good on paper. On the contrary, you may inadvertently narrow your future career choices (or at least it will plant a question in the interviewers' heads). Being overly aggressive in jamming your schedule during b-school may also interfere with your availability in networking, which I personally consider to be one of the most important reasons to go to business school.