If you really want windows messenger, you can use
gilly which almost has the same functions and can log you in with a shortcut to:
code:
"C:\Program Files\Gilly Messenger\GIlly Messenger.exe" me@hotmail.com mypassword
you can then use msn 6/7's autosignin to automaticly sign in to your account and gilly to sign in to another.
running two apps at the same time can be done using applications like
AppLoader