Of course it's not a legal or even an etiquette requirement for a witness to be an attendant. Not everyone even has attendants, and many people, especially brides and grooms who have children, have minors as attendants.
As a matter of fact, I usually see witnesses who are not attendants. It's a way to include more people with a special role. And at Jewish weddings, there are also 2 witnesses for the ketubah (Jewish wedding contract), too, so that's two more spots to honor someone. For the ketubah, in fact, the witness cannot be a relative, and as siblings and cousins are so frequently honor attendants, that means that that BM and MOH are in fact very rarely the witnesses.