Will you use one field with two check boxes or two fields with one check box each?
What if they select both check boxes? Say be mistake?
If the user is only supposed to select one of the two options, a radio button format on a single field would be the better option.
You can check the value of a single check box field or a radio button formatted field in a calculation just like you would a field formatted like an edit box. Fields formatted to permit selecting multiple values in the same field such as a group of check boxes requires a different approach.