Doing it right now. I have SQL 2005 and two SQL 2008 instances running.
It's easy to do. Just install SQL Server 2005, and when it asks you which instance, make sure you choose "Named Instance", and give it a name you will remember (such as, I dunno, SQL2005?).
Then, install SQL Server 2008. Same thing when it comes to the instance - make sure it's a named instance (Say, SQL2008). In your Services control panel you will see all the SQL stuff duplicated, but it will have the instance name after each entry (SQL2005/SQL2008).
When you need to connect, you connect to
machine\instance, so you would connect to
localhost\SQL2008 as the server name, and off you go.
It is definitely memory intensive, so you might choose to disable them if you're not using them right at the moment. Under idle with nothing happening right after bootup they're using about 150mb each.