Holding more than one post or office at the same time (often expressed as 'to concurrently hold X').
彼は大学で教員と学部長を兼任している。
He is serving concurrently as a professor and as the dean of the faculty.
小さな会社では一人が営業と経理を兼任することが多い。
In small companies, one person often handles both sales and accounting.
その部署の責任者を兼任することになった。
I've been assigned to concurrently serve as the head of that department.