From the point of the net protocols, yes, they can.
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.