About 2 PCs play GW

I have 2 GW Accounts and 2 computers connect with router. These 2 computers can read web site together but only one can play guild wars. What should I do to make 2 PCs play Guild wars at the same time?

Well I've had 3 different accounts logged in from different computers, and there has been players who have had 2 accounts on with 1 computer, so it shouldn't be an IP adress restriction.

Does the router come with a firewall, or some other form of port forwarding? What brand and model is the router?

The router is Buffalo air station G54(WYR-G54).

I don't think it's about the firewall since one PC is playing GW while this PC I use to write this post cannot play.

WYR-G54 has NAT and Firewall, and I'd assume NAT is turned on, to have multiple computers connected to one subscriber line.

I'd guess there's NAT or DMZ settings that forward the port GW uses to one computer, so the other one is left out. Check the manual of the router for more info, and check the port forwarding settings in the router (explained in the manual)