Yes, this will work. The LaserWriter should ignore the "other" port, and
only listen to the active port. The soft-switching capability
enable/disable the ports correctly, as will the DIP switches.
To our knowledge, there isn't any software on the market that automatically
takes advantage of this feature. If you want to take advantage of this
feature, you'll have to write the PostScript code (there are examples of
such code in the LaserWriter Owner's Guide).
There is a consideration: The LaserWriter needs to receive the code to
change modes from the system connected to the active port. This means that
both an MS-DOS machine and a Macintosh need to have the code, and each will
tell the LaserWriter to listen to the other computer.
All in all, a much better solution is to use a single interface for all
machines. LocalTalk is your best solution, with a LocalTalk interface card
installed in the MS-DOS system.