You can use hook_field_delete to determine which field is removed, and if its "yours" - find out which field you need to delete next and delete it

function hook_field_delete($entity_type, $entity, $field, $instance, $langcode, &$items) {
   if ($field['field_name'] == 'YOUR_FIELD_NAME') {
       //do some stuff to determine which
       //field you need to delete
       //and delete it 
       //how to remove the field - the link below
   }

}

how to remove the field

You can use hook_field_delete to determine which field is removed, and if its "yours" - find out which field you need to delete next and delete it

function hook_field_delete($entity_type, $entity, $field, $instance, $langcode, &$items) {
   if ($field['field_name'] == 'YOUR_FIELD_NAME') {
       //do some stuff to determine which
       //field you need to delete
       //and delete it 
       //how to remove the field - the link below
   }

}

how to remove the field