Internal audit requirements

" I would like to know who can be an internal auditor in the company? My company used to have a second party who did the internal audit. But It cost us too much so my boss has changed his mind and he is going to ask one of his employee to do the internal audit. As far as I know there should at least two people in the company to do the internal audit. One person can do all department a pat form hers and the second person can audit the first one's department. Am I right?”