From the point of the net protocols, yes, they can.
![Yes :yes:](http://www.ofpec.com/forum/Smileys/NewSet/yes.gif)
Each client connects through the NAT with an unique link to the server, but of course will be seen with the same source-IP, that is the router's one.
It's the typical situation of a LAN with access to the internet.
Anyway, as Gruntage wrote, a (generic) game could decide to disallow multiple connections from the same IP.
Speaking about OFP/Arma, the 2 computers will be logged as Player/ID/GUID (that haven't relationship with IP) and so will play regularly.
![Good :good:](http://www.ofpec.com/forum/Smileys/NewSet/good.gif)