How best to share data between two offices?

I have a client with an office in two different countries.  Each office has a QNAP TS-219P+ NAS drive which holds the office's network drive shared data, which is shared between 5 PCs in one office and 3 PCs in the other.

Office A's 5 PCs can access NAS A's network drive locally
Office B's 3 PCs can access NAS B's network drive locally

For Office A to access NAS B, or Office B to access NAS A they use FTP or the web interface.

We now want both offices to be able to access the same data.  I doubt that's going to be possible with the TS-219P NAS units, and we're happy to junk them and start again, but with what?

Office A has a 16 Mbps download, 1 Mbps upload internet connection
Office B has a 30 Mbps download, 5 Mbps upload connection

There's around 20GB of data on each NAS, all Word/Excel/Powerpoint/PDF documents but of course little changes day to day.

Currently we back up to an external hard drive that is taken offsite, and to an Amazon S3 account.

We'd spend about GBP 2000 (US$3500) without worrying about it, but if there are options more costly that would work better, we'd consider it.

Any ideas?
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Are you in a domain or work group environment? This could be easily managed by creating VPN tunnel between the sites, so both sites will virtually be on the same network. All you might need is VPN capable routers on either side. But the upload speed worries if the files tend to be bigger.
RedLondonAuthor Commented:
No domain, just 5 Windows 7 PCs in one office and 3 in the other.  Some of the Powerpoint presentations, especially when nearing completion, are 30-35MB though.

The slow internet connection speed pushes me towards keeping local copies of files in each office, but how to keep them in sync is the problem with that.
Ben HartCommented:
It's a little obscure but you could setup a small linux box in each site, and possibly rsync between the two using DDNS.  NAT 443 then mount the nas boxes to your linux machines.
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RedLondonAuthor Commented:
The QNAP boxes run Linux and allow one-way sync, either scheduled or realtime, but that'd only mean Office B can see Office A's files: if they made changes and saved them to their local copy the changes would be overwritten on the next sync, or if they saved the changed files to their own drive, we'd end up with two different copies of the same file.
Bryant SchaperCommented:
Do the sites have IP connectivity between each other, ie a computer on a can access a share on b and vice versa?

Then I would say rsync as well, a site has a server you can use a program like watch directory to main them too.

Backup spec on model:

Backup Solution

USB One Touch Backup (Import/Export)
Apple Time Machine Support with Backup Management
Block-level Remote Replication
Work as Both Rsync Server & Client
Supports Encrypted Replication between QNAP NAS Servers
Back up to External Storage Device
Back up to Cloud Storage (Amazon S3)

You are more familiar with the model, but you could use a netgear readyNAS

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Semper PhiSystems IntegratorCommented:
This VPN Router solution would work well.

Product: Kerio Control

and they have partners that can assist with setup / config
RedLondonAuthor Commented:
The new QNAP TS-251 models have a specific two-way sync feature which can run every 5 minutes.  That means both offices have a local copy of all files, and any changes made in one office appears in the other.  In the unlikely but inevitable event that a change happens at both locations, the first file is renamed and the second stored beside it so no data is lost.
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