you need to set the ControlToValidate and ControlToCompare to the ID of the controls you want to compare. There is documentation on it here[^], the main things you need to set are the Type property (seeh here[^]) and the Operator property (see here[^]) as well as the various display stuffm such as the error message.
Totally misread your question, sorry! I think it validates of either is changed, but you've sown the seed of doubt in my mind! It wouldn't make sense to validate on only one control...