It looks a little confused by your syntax. Why not put them in two different selects (if that's possible...)?
I don't know if this will help, but I found this, it will prevent it from adding the backticks altogether.
$this->db->select() accepts an optional second parameter. If you set it to FALSE, CodeIgniter will not try to protect your field or table names with backticks. This is useful if you need a compound select statement.
http://ellislab.com/codeigniter/user-guide/database/active_record.html