By default the it is forbidden to login to the same account twice at the same time (the session is locked to username/password combination)

The only way how to do ensure this is to use a little trick:

1. Create userA@domain.com and leave his mailbox path on a default value (domain.com\userA)
2. Create userB@domain.com AND change his mailbox path manually to value domain.com\userA

So you have two users with two username/password BUT they are in fact going to the same mailbox.