I have 2 tables order_retailer and order_customer
they both have auto incremented primary key order_id
To keep an order id unique in the whole system I want
No it is not possible to set it table wise.
Mysql has 2 variable auto_increment_increment and auto_increment_offset and values of both these variables are 1 by default. If you change any of the value it will have global effect.
To solve your problem create a stored procedure which will according to needs of having even id.
Refer this link http://dev.mysql.com/doc/refman/5.0/en/replication-options-master.html#sysvar_auto_increment_increment
Hope this helps