EC2 instance communication

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http://serverfault.com – I currently have one EC2 instance that is starting to send alerts for high CPU usage. I want to create another instance and use this instance for the database and keep my existing instance for the web server. I'm running Windows Server 2008 R2 and have an Elastic IP for the current server. I want to install another instance, install SQL Server express and allow the instances to communicate using SQL Server, however I don't want SQL Server to be open to the rest of the internet, just the web server instance. Can anyone give advice on how to easily do this? (HowTos)