Based on past discussion on being able to swap between characters on the same account without logging out, this would probably be impacted by the same constraint that has resulted in CCP not reworking the codebase up to now.
According to CCP Karkur (in relation to swapping between characters on the same account):
Once a connection has been made, both the server code and client code make a lot of assumptions that the characterID will not change, and it’s very far from trivial to change that (otherwise we probably would have done that a long time ago)
That same issue will exist, if you wanted to switch between two characters on different accounts. The characterID would need to change for that to happen.
Unfortunately then, the 64 bit client alone couldn’t resolve that, since the server side code would need to be rewritten too.
It’s not that CCP don’t want to do it, but that because of the amount of work involved, it just doesn’t receive any priority.