You may only be logged into a single account once. If you try to log into it again on another computer or through another running client (multi-box), while still logged in to it, you'll get kicked off the server.
In order to play more than one character at a time, you'll need separate accounts for each instance that you want to run. So, if you want to log into 2 characters at the same time, they will each need to be on two separate accounts.
I do not know any MMO that allows you to log into the same account more than once per session.
Keep in mind, almost all game settings are stored on the server. So if you change some settings on one computer, when you log into that same account on another computer, the settings will have been changed to the last made changes.