Oh well, it was worth a shot.
Looks like your options are:
1-override the validate() method of your ActionForm. Manually code the logic for this validation, then call super.validate() to handle the validation framework validations.
2-Create a custom validation rule that will handle this situation.
This link shows you how to create a custom validator rule.
[ July 30, 2006: Message edited by: Merrill Higginson ]