$where="(`shopid` = '10' OR `shopid` = '11')";
$ this-> db-> где ($ where, NULL, FALSE);
и для условия AND используйте
$this->db->where('shopid <>', '18')
т.е.
$where="(`shopid` = '10' OR `shopid` = '11')";
$this->db->where($where, NULL, FALSE);
$this->db->where('shopid <>', '18')