If you have two columns, your code will also need to ensure they're exclusive. I doubt it would be much use to you if both are checked ... Aside from requiring less code, and simplifying the document, that's one reason I prefer to use a single dropdown for such scenarios.
For some examples you might find useful, see:
https://www.msofficeforums.com/word-...ents-form.html
https://www.msofficeforums.com/word-...own-field.html
https://www.msofficeforums.com/word-...html#post33489
https://www.msofficeforums.com/word-...html#post55995