One field with both Single and Multiple value by content type

Interesting Request of the Week: 

We have a Departments taxonomy that we use on all our content types to relate the content to a particular department or departments.   It supports unlimited choices.

We also have a Departments content type that also should contain the same departments taxonomy (so we can link the node and the term).

The Challenge: We want the Departments taxonomy term-reference field on the Departments content-type to only allow selecting a single value, since the node and the term are a 1:1 relationship.