AJAX functionality is broken

My custom module adds an ajax functionality for a select list, which is dependent on another field.
$form['field_fieldone'][$langcode]['#ajax'] = array(
'callback' => 'mymodule_callback',
'wrapper' => array('replace_div_1'),
);
and so on
$form['field_fieldtwo'][$langcode]['#prefix'] = '

';
$form['field_fieldtwo'][$langcode]['#options'] = _ajax_get_fieldtwovalues_options($parameter);

Everything works, with cardinality unlimited too.
But if I switch from widget "select list" to your widget "multiple selects list" (fieldtwo), the ajax functionality is broken, nothing happens with options fieldtwo.
And I can not add a second delta value !

What is wrong ?