unique index - cakephp isUnique for 2 fields?

Question

I have a registration form in which users can fill in two email address (email1 & email2). Marketing's requirement is that they need to be unique (unique as in if we had 10 users, then there would be 10*2=20 unique email address).

The system is already built on cakephp, so what I'd like to know is, is there something similar to the isUnique feature (unique in one field) that can do this right out of the box? Or am I doomed to code this myself? Thanks in advance.

EDIT: built on Richard's example, this worked for me:

function checkUnique($data, $fields) {
    if (!is_array($fields)) {
        $fields = array($fields);
    }
    foreach($data as $key) {
        $checks = $key;
    }
    if (empty($checks)) {
      return true;  //allow null
    }
    foreach($fields as $key) {
        $tmp[$key] = $checks;
    }
    if (isset($this->data[$this->name][$this->primaryKey])) {
        $tmp[$this->primaryKey] = "<>".$this->data[$this->name][$this->primaryKey];
    }
    return $this->isUnique($tmp);
}

Answer 1

I posted a solution to this on the CakePHP Google Group:

http://groups.google.com/group/cake-php/browse_frm/thread/b3a1e4ae3eeb6091/e168f54bac27c163?lnk=gst&q=checkUnique#e168f54bac27c163

Add the following to your AppModel:

        /** 
         * checks is the field value is unqiue in the table 
         * note: we are overriding the default cakephp isUnique test as the 
original appears to be broken 
         * 
         * @param string $data Unused ($this->data is used instead) 
         * @param mnixed $fields field name (or array of field names) to 
validate 
         * @return boolean true if combination of fields is unique 
         */ 
        function checkUnique($data, $fields) { 
                if (!is_array($fields)) { 
                        $fields = array($fields); 
                } 
                foreach($fields as $key) { 
                        $tmp[$key] = $this->data[$this->name][$key]; 
                } 
                if (isset($this->data[$this->name][$this->primaryKey])) { 
                        $tmp[$this->primaryKey] = "<>".$this->data[$this->name][$this- 
>primaryKey]; 

                } 
                return $this->isUnique($tmp, false); 
        } 
} 

and is used in your model validate:

        var $validate = array( 
                "name"=>array( 
                        "unique"=>array( 
                                "rule"=>array("checkUnique", array("name", "institution_id")), 
                                "message"=>"A contact with that name already exists for that 
institution" 
                        ) 
                ) 
       );