Zend FR

...

/* Email du commercial */
$poste_commercial_mail = new Zend_Form_Element_Text('poste_commercial_mail');
$poste_commercial_mail->setLabel('Email du commercial');
$poste_commercial_mail->addValidator('EmailAddress', true, array(
    'message' => array (
        'test'=>Zend_Validate_EmailAddress::INVALID) 
    ));      
$this->addElement($poste_commercial_mail);

...

$poste_email = new Zend_Form_Element_Text('poste_email');
$poste_email->setLabel('Email de contact');
$poste_email->addValidator('EmailAddress', true, array(
    'message' => array (
        'test'=>Zend_Validate_EmailAddress::INVALID) 
    ));  
$valid = new Mylib_Validate_AtLeastThisField('poste_commercial_mail'); 
$poste_email->addValidator($valid);  
$this->addElement($poste_email);

<?php
require_once 'Zend/Validate/Abstract.php';

class Mylib_Validate_AtLeastThisField extends Zend_Validate_Abstract {
    
    protected $_contextKey; 

    const BOTH_EMPTY = 'bothFieldEmpty';
    const FIELD_NOT_FOUND = 'confirmationNotFound'; 

    protected $_messageTemplates = array(
        self::BOTH_EMPTY => 'Both field are empty',
        self::FIELD_NOT_FOUND => 'No field check was submitted'
    );

    /**
     * Constructor of this validator
     *
     * The argument to this constructor is the third argument to the elements' addValidator
     * method.
     *
     * @param array|string $fieldsToMatch
     */
    public function __construct($contextKey) 
    { 
        $this->_contextKey = $contextKey; 
    } 

    /**
     * Check if the element using this validator is valid
     *
     * This method will compare the $value of the element to the other elements
     * it needs to match. If they all match, the method returns true.
     *
     * @param $value string
     * @param $context array All other elements from the form
     * @return boolean Returns true if the element is valid
     */
    
    public function isValid($value, $context = null) 
    { 
        if ((null === $context) || !is_array($context) 
            || !array_key_exists($this->_contextKey, $context)) 
        { 
            $this->_error(self::FIELD_NOT_FOUND); 
            return false; 
        } 

        $fieldValue = $context[$this->_contextKey]; 
        if (empty($value) && empty($fieldValue)) { 
            $this->_error(self::BOTH_EMPTY); 
            return false; 
        } 

        return true; 
    } 
    
}

// Envoie du formulaire
if ($this->getRequest()->isPost()) {

    $formData = $this->_request->getPost(); // Récupère les données envoyées
    
    if($form->getValue('poste_commercial_mail')== ''){ $form->poste_email->setRequired(true); } // ajout du validateur si le champ 'poste_commercial_mail' est vide.