[mysqld_multi]
mysqld = /usr/bin/mysqld_safe
mysqladmin = /usr/bin/mysqladmin
user       = multi_admin
password   = multipass

[mysqld1]
user    = mysql1
pid-file = /var/run/mysqld/mysqld1.pid
socket = /var/run/mysqld/mysqld1.sock
port=3307
datadir=/var/lib/mysqld1
server_id=4
log-bin=mysql-bin
binlog_format=mixed

[mysqld2]
user = mysql2
pid-file = /var/run/mysqld/mysqld2.pid
socket = /var/run/mysqld/mysqld2.sock
port=3308
datadir=/var/lib/mysqld2
server_id=3
skip-log-bin

Reporting MySQL servers
MySQL server from group: mysqld1 is running
MySQL server from group: mysqld2 is running

$ mysql -u root --socket=/var/run/mysqld/mysqld2.sock -p
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: YES)