versi 2

<?php
$data = array();

/*
versi 1 pake ini
->join('user usr_nilai', 'n.id_ygdinilai = usr_nilai.id_user')
->join('user usr_guru', 'n.id_penilai = usr_guru.id_user');

*/
$this->db
->where('n.id_ygdinilai = usr_nilai.id_user')
->where('n.id_penilai = usr_guru.id_user');

$this->db->order_by('n.id_nilai', 'desc');
$this->db
->select('n.*, usr_guru.nama guru_nama, usr_nilai user_nama')
->from('nilai n,user usr_nilai,user usr_guru');

$Q = $this->db->get();

if ($Q->num_rows() > 0) {
    foreach ($Q->result_array() as $row) {
        $data[] = $row;
    }
}