In some areas, it's username; in other areas, it's user name. For example, in the phrase usernames_must_be_unique, it's usernames, but in the phrase commas_not_allowed_in_user_names, it's user names. Obviously, it's like this with various other phrases.
Both spellings are correct, but it's just a matter of consistency. Everything should be either username(s) or user name(s), but not both. In addition, the phrase titles would need to be updated to reflect whatever change is done.