So you have two computers that are not in the same place, and you want them to share the same synchronised folder, communicating directly with each other.
A 6 minute screencast
showing how to share files between your computers in different locations,
such as home and work.
You can add even more computers using the same method shown here.
If you have a laptop that is sometimes near another computer, you can speed up file transfers when it is by also connecting it using the local pairing walkthrough.