I have not tried this. However, in a SQL Server 2000 cluster, when you go to install another instance, it requires a new virtual name. If there is already a default instance, you must install a named instance (only one instance can listen on 1433 if the instances end up on the same physical node). I assume that hasn't changed for SQL Server 2005. You might try and send a private message to Brian Knight... he's a SQL Server MVP and knows far more about SQL Server clustering than I do.
K. Brian Kelley
@kbriankelley