Arc allows two accounts to be logged in simultaneously, but only allows one computer on each account, and no more than two accounts. Because Arc is also the place to redeem Codes such as Jones Blessing, you can redeem each code twice; but you have to log out of one account and into another.
The trick to signing in to Arc is to enter both, your email address, AND your account name, in either order. It says it wants one or the other, but the first will not work; they always give an error message. Very disturbing when you have spent a long time on characters and do not want to lose them.
Arc is not required to play the game (it can be removed with the Windows Task Manager- End Task - and PWI element clients keep running), but if there is a way to log in without it, it is not known by current Wikia contributors. Please add that information if you know it.