If you find yourself constantly juggling multiple development environments, you'll no doubt come up against the issue of the Developer Toolkit only supporting a single installation – either for CRM 2011 or CRM 2013.

To overcome this, I have been using the CRM 2013 version again CRM 2011 with no issues. The only thing to remember is that if you create a new project that needs to target CRM 2011, you must remove the version 6 assemblies from your plugin and workflow projects and re-add the version 5 ones. You can do this using nuget with a specific version:

Install-Package Microsoft.CrmSdk.CoreAssemblies -Version 5.0.17

See my post http://develop1.net/public/post/Manage-your-SDK-assembly-references-the-easy-way.aspx for more information.

@ScottDurow