Welcome to network programming. You'll need to use client/server architechture.
The clients are just front-end programs, and the server is the real meat that will have all the information needed. When a client needs to know something, it should ask the server. And when a client does something, it needs to let know of the server.
If you're using MFC, you could use the socket classes provided by MFC -
CSocket
and
CAsyncSocket
(see, for instance,
Windows Sockets in MFC). There are many articles on socket programming on this site. Just do a search, and readup on that. Post a query here if you get stuck somewhere.
Hope that helps. :)