Rule number1. Take good backup before upgrade.
Test your upgrade plan in test environment with the same setup i.e. two masters ideally using snapshot of production databases (if possible).
If you have additional servers in PROD. You could minimize downtime. For instance add two nodes (C and D) into existing setup i.e.
-- CURRENT_Setup
A_5.1 <-> B_5.1 # bi-direction replication, multi-masters
-- Extend original step
A_5.1 -> C_5.5 -> D_5.5
C_5.5 <-> D_5.5 # setup bi-direction replication between C AND D
So that you can failover to C and D
Read below article to understand configuration/server changes.
http://dev.mysql.com/doc/refman/5.5/en/upgrading-from-previous-series.html
Also, please don't just upgrade RPMs (binary upgrade). To upgrade to 5.5 from any previous version, MySQL recommends that you dump your tables with mysqldump before upgrading and reload the dump file after upgrading (and run mysql_upgrade script). Use the --all-databases option to include all databases in the dump. If your databases include stored programs, use the --routines and --events options as well