HTML_QuickFrom - How to do Group Element validation?

Using HTML_QuickForm how do I apply a group validation on two elements when there is no other validation errors? I'm trying to display one error message when username and/or password have a mismatch (invalid). I still want it to highlight each element within the group that has an error. Right now I'm trying to achieve this using AddFormRule(). However it's not working as this rule reports an error when technically it should not be an error until both username and password pass the other validation first (each field have a value, each field's value fit within the range length).

Any help would be much appreciated. If you require more information please let me know.