Are you looking for radio buttons rather than check boxes?
Just a thought...
Try using radio buttons instead of check boxes.
Just as a followup to my previous comment, you need to remember that radio buttons do not guarantee complete control. By shift-clicking you can have have both radio buttons activated which may be a problem for you, so to make certain that only one button is filled instead of two, you should use a script trigger upon exiting the field or committing the record to verify it.
Radio buttons worked! I figured it was something simple, I guess the check box is a force of habit kind of thing.