This week's book giveaway is in the OCAJP forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide 1Z0-808 and have Jeanne Boyarsky & Scott Selikoff on-line! See this thread for details.
Struts validation framework was designed to handle only basic, simple validations. Once you get into more complex scenarios, you generally have to write your own validation logic, either by extending the validate method of your ActionForm (you can still call super.validate() to have the rest of the framework validations performed), or by writing a custom validation rule.
Having said that,you might be able to use the validWhen rule for this. See the section entitled Designing Complex Validations with ValidWhen in this link.