Until such time as there is a web ui for this, you can sync them from a remote machine with push/pull commands.  Assuming that URL1 and URL2 represent the two remote machines, you would do the following:

<pre>
fossil clone URL1 syncup.fossil
fossil pull URL2 -R syncup.fossil
# More pulls from other repositories here if more than 2
fossil push URL1 -R syncup.fossil
fossil push URL2 -R syncup.fossil
# More pushes to other repositories here if more than 2
rm syncup.fossil
</pre>

Not much additional network traffic except for the first clone and if the <tt>syncup.fossil</tt> repository is kept around between syncs instead of <tt>rm</tt>ing it (subsequently pulling from URL1 rather than cloning from URL1) network traffic really won't be all that large.  (Of course suitable users and passwords with appropriate privileges will have to be used.)