I think he means logging in. absolutely you can... a friend of mine plays on my machine all the time. all he has to do is log in normally with his account data. no key entry or anything else... Just e-mail and password. and he is in his account. the reverse is true as well at his place. Once you put in your key to your account it follows you around no matter where you log in because that key is stored on arena-nets servers not your machine.
So if you want to go to a friend house and he has a few computers and you want to have a online party, you don't need a thing. just log-in normally and your good to go. Heck I was playing it in a cyber cafe the other day and everyone was like WTH is that? COOL!!! I must have signed up 12 people that day through Arena-nets online store. lol Then all they had to do is go home download the client from the web-page and log in. simple as that.
Yes Dargon is correct, 2 accounts installed 1 under 2 different user profiles in XP and swapping through the XP fast swapping tool you could bounce back and forth between 2 accounts. Would be a pain in the ass, but doable.
if you had 2 nic's on a computer with dual monitors and 2 gw accounts (or games installed) you should be able to run both GW's from 1 computer. or am i mistaken ???