Custom form validator for checking more than one field

Dear Perchers,

I am trying to put together a custom form validator for a form field that is required depending on whether another form field is checked. Is it possible to check two or more fields with a custom validator? Or the number of selected checkboxes? If not, what solution would you suggest?

This is the helper I am trying to use to make the title required if the abstract is poster:

<?php
    class Reg_Validator 
    {
        public static function check_abstract_title($abstract,$title) 
        { 
            if (($abstract=="poster") AND ($title=="")) { 
                return false;
            }  
            return true;
        }
    }
 ?>

Then in the form:

<perch:input id="abstract" type="radio" options="lecture,poster,none" />
<perch:input id="title" type="text" helper="Reg_Validator::check_abstract_title" />
<perch:error for="title" type="helper"><span class="error">You need to provide a title for the poster.</span></perch:error>

Thank you very much!

Laszlo