You should refer to chapter 3.4 of the Sitecore 7 Scaling Guide which will suggest you to use Microsoft Distributed File System (as Jens pointed out) or the Web Deployment Tool.
I have implemented the Web Deploy solution a few times and that works pretty good, but it can be quite difficult to setup.
The DFS solution wasn't part of the Scaling Guide the last time I had to implement file synchronisation, so I can't comment on that.