The trick is as follows:
- Add the file you wish to reuse to a WinMD class library project (e.g. "MyWinMdProj")
- Mark the resource as "content" in its property page (usually is by default)
- Add a reference to the class library from the top-level Metro app
- Use a script tag with the ms-appx protocol to reference the file, i.e.
< script src="ms-appx:///MyWinMdProj/path/to/file.js" >
Note the three slashes; "MyWinMdProj" is a folder relative the Metro app's package (i.e. ms-appx:///) as opposed to being its own top-level package.
Note that any file included in this manner will be exposed on the filesystem under C:\Program Files\WindowsApps - but all of your html, js, css, etc. files from the top-level app project already are anyway, so you're not any worse off in that regard than if you copied and pasted the code in between two (or more) different apps.