There's a slight problem though - the VS Express C# IDE has, by default, disabled the build configuration and platform settings to present a simpler interface to beginners. In order to use this project template you need to re-enable these features and should probably have a reasonable understanding of what they do. This could be the reason that the XNA devs have mentioned that building for both platforms in one project is not currently supported.
But, if you do understand the build configurations in VS and think this might be useful then you can find out a little more and download the project template here.
Cheers,
Leaf.