La notte porta consiglio.....
For MyISAM and BDB tables you can specify AUTO_INCREMENT on a secondary column in a multiple-column index. In this case, the generated value for the AUTO_INCREMENT column is calculated as MAX(auto_increment_column)+1 WHERE prefix=given-prefix. This is useful when you want to put data into ordered groups.
http://dev.mysql.com/doc/mysql/en/ex...INCREMENT.html

prova come spiega il manuale. Forse potrebbe essere la giusta soluzione per il tuo caso.