Drobo is ill advised, because it is so slow.
If you want a NAS, in contrast to a much faster but also more expensive SAN, there are two brands that stand out IMO, QNAP and Thecus. The QNAP TS-879 Pro is pretty expensive, the Thecus N7700Pro is much more affordable and both are really good. You can connect them with dual gigabit connections, much better and faster than FW800.
Keep in mind, like Jon pointed out, that a NAS is for storage and accessability, not for video editing. For editing you need a much faster solution.
BTW, I use the Thecus N7700Pro myself, and it has been very reliable, provided you have reliable disks installed.
I'm sure that Haarm Millard will give you a more articulate reply, but NAS drives just don't have the transfer speeds or bandwidth required for video editing.
The very minimum spec is for 7200 rpm drives, and for many formats now, it's raid-0.
How about an external esata raid-0 such as the G-Raid Micro. Just hot swap it between the two units.
Well, I'll be dipped in dogs**t. He beat me to it ;-)
See - I told you.
Oops - and I put the double "aa" in the wrong place! Sorry.
For me personally I'd just buy a mediasonic enclosure that can hold 4 sata drives, then purchase 4 1TB 7200RPM drives. Then hook it it up using e-sata and run it in Raid-5 so you have the speed of raid-0 but you also have data security. It would cost you around $500usd for everything and you guys could just hot-swap between the two units. Just like Jon mentioned.
Put a fast, large cache controller w/ lots of drives in one of the PCs. I used a 4GB cache Areca model w/ 8 x 1TB drives.
Also on that PC, you would probably be best off with a fast quad-core (or better) Intel cpu and at least 24GB of RAM. (and Win7 Pro, Enterprise, or Ultimate)
And on each PC, have additional drives for scratch, cache, and render outputs.
I've actually played with somthing like this setup and it works remarkbly well. I don't believe that Adobe supports running this way - "over a network".
Why is this cheaper and easier than NAS, SAN, etc.? Because you only need to go across the 1Gbit LAN from a single PC to another single PC which helps eliminate "shared" access performance.
Why not connected storage like others are mentioning? Because you both need to work on different projects at the same time.
moderator - should this be moved to HW forum?