How to upgrade MySQL 5.0 to 5.5 with master-master replication?

I have to update 2 hosts with SUSE Linux Enterprise 11 from SP2 to SP3 which implies an update from MySQL 5.0.94 to 5.5.31.

The release notes state:

SLE 11 SP3 introduces the upgrade of the MySQL database to version 5.5. This upgrade involves a change of the database format and the database needs to be converted before MySQL can run again. Therefore MySQL is not running directly after the upgrade. To migrate the MySQL database, run following commands as root:

touch /var/lib/mysql/.force_upgrade

rcmysql restart

Now the tricky part is that both hosts are configured with a master-master replication. Read access should be possible during the update, write access is not needed. How do I do this?