Outlook Express is not available on Windows Vista.
Windows Mail is not available in XP.
Windows Live Mail is available for both platforms, which is the replacement for the two aforementioned programs.
You can do what you' want but you'd have to set up a third computer and have both machines (with Live Mail on them) point to the same location. Alternatively point one machine to the other.
Then of course the issue you'll have is if both clients were open at the same time. File access clash = potential corruption.