This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Method 1: override the validate method on your form bean. In that method, you would first call super.validate() to perform the validation framework validations, and then write your own logic to compare the two fields.
Method 2: Create your own validation rule that compares two field. Fortunately for you, the Struts Validator Guide provides an example of the very thing you're trying to do in the section entitled Pluggable Validators.