Zend Form Validator : Element A or Element B

I have two fields in my Zend Form, and i want to apply the validation rule that ensures the user enters either one of the these two fields.

    $companyname = new Zend_Form_Element_Text('companyname');
    $companyname->setLabel('Company Name');
    $companyname->setDecorators($decors);
    $this->addElement($companyname);

    $companyother = new Zend_Form_Element_Text('companyother');
    $companyother->setLabel('Company Other');
    $companyother->setDecorators($decors);
    $this->addElement($companyother);

How can i add a validator that will look at both fields?