The controls in the view reflect the current state of the data, and allow the user to interact with it. So if two of the radio buttons have the same value then they will both show as selected when the field takes that value. I'm fairly sure there's no way to prevent that behavior and in some ways it wouldn't make sense (for example, how would InfoPath know which button to show as selected when the form is submitted, closed, and reopened)?
Have you considered deleting one of them and giving the remaining one prompt text that combines the two radio buttons you have right now?
The alternative would be to give them distinct values and put a rule on the radio buttons' field to populate a non-displayed field with the actual values you want to use.