The Windows Phone 7 Tools, including the latest Mango update, provide no supported way for running two emulators at the same time.  This can be a problem when trying to see how a multiuser application works across multiple devices.  Fortunately there’s a way to workaround this limitation using some unsupported edits to your WP7 emulator settings.
NOTE: This is not a supported change and if something breaks on your machine, well… you’ve been warned :-)

Step 1: Open explorer in Administrator mode and go to C:\ProgramData\Microsoft\Phone Tools\CoreCon\10.0\addons

Step 2: Copy “ImageConfig.en-US.xsl” and name it “ImageConfig.en-US 2.xsl”

Step 3: Open this new file in your text editor of choice and locate the DEVICE element and change the device name to something of your choice and the device GUID to a new GUID as shown.  Use the Online GUID Generator to make generating new GUIDs easy.

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Step 4: Given the WP7 is a virtual machine, we also need to supply a new Virtual Machine ID, so locate the VMID section and provide a new GUID in there as well.  Again the Online GUID Generator can be used for this purpose.

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Ensure that the GUID you insert is in the same format (i.e. with the braces)

Step 5:  Save the file and start the Application Deployment Tool.  You should now see your new emulator available to deploy your applications to:

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If you close and restart Visual Studio 2010 you should also seen the second phone emulator available as a deployment target:

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Note that of you run a phone application in debug mode there seems to be no way to launch a second instance on the other emulator whilst the phone is running.  It’s not a major problem though since you can simply take the debug .xap file and deploy it to the second emulator using the deployment tool.

I hope this helps you out.  Enjoy!