Make sure that in both Visual Studio 2010 and Visual Studio 2012 the same work space name is selected. If you cannot see the workspace of 2012 in Visual Studio 2010, check the advanced workspace settings of the workspace in 2012. Ensure that the workspace type is set to server.
When both Visual Studio versions are using the same workspace name, they should be able to see and use each others file mappings as well.
This is what the settings in 2012 should look like:
And that workspace should be visible in 2010 as well:
Make sure you select the same workspace name in both versions of Visual Studio: