We've had our setup for almost 2 years now and it's taken until now for someone to notice that when 2 people open the same file on 2 different servers in a namespace, the last saved is given priority and overwrites the first.
Ideally we need to resolve this, but I'm trying to understand the concept and getting myself even more confused!

So, our setup is thus.

2 sites, physically connected via fibre.
DC on each site, although in the same subnet (for the purpose of this example They also share a VLAN, vlans here set up by type rather than location (servers, printers, wireless etc etc). Clients are split, so clients at site A get one range of addresses and site B get another range. Routing and IP helpers dictate where they get their DHCP and DNS information from though.

DFS namespace (and replication but that's irrelevant here) is set up on our student and staff shared areas with a server on each site. The ideal scenario, and the way I assumed it would work, is that it routes via "lowest cost" - therefore theoretically the clients at site A connect to the server at site A for those shares, and the clients at site B to server B.

However that doesn't appear to work, they connect to whichever server they see fit.

I know DFS works in conjunction with the costings etc from AD sites and services. Currently we have 2 sites with the DC in each set up appropriately. However the servers being in the same subnet put them in the same site.
I added a new subnet in AD S&S for each server alone - for instance and and popped those "subnets" in each site. DFS management picked that up and each server was then in the correct site - woo! However, DFS still wasn't playing ball and what server clients attached to didn't seem preferential. (Maybe we didn't give it enough time to replicate? As things were live we put it back to how it was!)

One thing that keeps cropping up on MS forums with little to no explanation short of pointing me at those ruddy Technet forums (By programmers, for programmers, understood by noone except those MS employees from India) is the costing under "Site Links".
Currently we have a single (the default) site link. Do/should I set up another link? Could someone briefly explain those links for me? All examples seem to deal with WAN which this isn't. Or am I barking up the wrong tree?