QF: Add javascript validation using addFormRule?

Hi I need a custom validation rule involving two separate form fields which is not a problem if I do server side validation using addFormRule() with my custom PHP validation function as callback. Is it somehow possible to add custom client side scripting? If I have both the custom php function as well as the custom javascript function - how can I add both of these to the existing form validation rules? Cheers, Tommy Ipsen