As of patch 1-0-31, logging in twice is now ... impossible? I'm sure some geek will prove me wrong, but it's much, much harder now. Before it would just boot you after detecting that you have two tabs open, but now it won't even let you login and will "Fail to join game" Diablo 2 style. Clicking the camp button will also immediately check you out of the game, whereas simply hitting F5 (refresh) will cause you to be stuck in the game world for about 30 seconds. So, for this reason, I recommend getting in the habit of camping out.

This was done because in the past, double logins could screw with the game data and actually cause loss of data in some cases.