As I was giving a demo of the new multi-query capabilities the other day, I was asked how I had four Instances installed on one small, underpowered laptop. In fact, I don't - I have one instance of SQL Server 2005 and one Instance of SQL Server 2008 installed.
What I've done is to create two "aliases" to the same Instances I already have. You can do the same - all you have to do is open the Configuration Manager and navigate to the "SQL Native Client Configuration" node. From there click the "Aliases" object and then right-click in the open pane. Select "New Alias..." and then enter a unique name and select a protocol, address and the same Instance name you have installed. I usually pick a different connection method (such as TCP/IP or Named Pipes) than I connect with normally (Shared Memory), so that I can test network issues anyway.