If there's a real reason to not use a one-to-one mapping, that's the way to do it.
Here it looks like you're mapping from departments to a department head (user). That just means a department can have only one head, not that a user can't have more than one department.
To paraphrase
Java Persistence in Action:
[...] you declare that a particular user can be referenced by at most one department,
as a department head.
(Emphasis mine.) I translate that to mean that a department can have only one head, not that a user can't have multiple departments. But I might be misunderstanding what you perceive the symptom(s) of the problem(s) is.