No synchronization tool can synchronize files that are open w/o running the risk of making inconsistent copies. Unless the tool has hooks into the application holding the file open to request that it "quiesce" the file there will always be a risk that a copy made of an open file will end up being inconsistent and unusable.
It sounds, to me, like you're going to be served poorly by just about any tool, given the profile of open files that you're talking about. I wonder if a version control system / document management system wouldn't possibly be a better fit for you.
I've used the SureSync synchronization tool from Software Pursuits, albeit not in the scenario you're distribing, and have been very pleased with it. It runs as a Windows service on the servers in the replication set and does delta transfers (with the "SPI Agent" add-on). It can replicate open files (can can quiesce VSS-aware applications), though you could potentially run into consistency issues, as I said above.
Response re: comments:
This is the classic fast/cheap/good triangle tradeoff. If you want your replicas to stay in sync throughout the day you're going to need to shell out a lot of money for fast connectivity. If you don't care that the replicas fall out of sync (but "catch up" overnight) then you can spend less money on fast connectivity.
I don't have any Customers who expect all files replicated in such a a manner to be "in sync" at all times on all servers. They don't have the money to spend on LAN-speed WAN connectivity to support it.
If you have a small corpus of files that need to be kept in sync more rigidly you could look at using this more real-time replication solution to cover those files and cover the rest of the files in a slower, less bandwidth-intensive replication solution.
You have to pay the piper somehow is, I guess, what I'm saying.
Best Answer
Create an account with the same username and password on both systems. Make a share on the one being replicated to. Run robocopy /Mir in a scheduled job under that user account. Bam, poor-man's replication.