IF you did everything right when installing AC=DC there should be a /2 folder in your ToD AC folder. IT is supposed to have a copy of all the files from AC folder in it to run AC. if it doesnt or there is no /2 folder then you did something wrong during the install.
typically after after every major installation i do a reboot of the computer just to ensure all things are working correctly.
the error message you are getting suggests that ac-dc isn't using the /2 folder to open/run the 2nd account and thus chokes since account 1 is using the files in the ToD AC folder.
-----signature-----
Ag Nar
Portal Mule Master of the Loresraat
Mastering the pixels of gamedom, 1 MMORPG at a time..