help - simultaneous accounting

I want to allow simultaneous use of accounts. However, *RFC 2866 *says that the* Acct-Session-Time*

can only be present in Accounting-Request records where the Acct-Status-Type is set to Stop [Page 17]

Does this mean that RADIUS protocol can not manage simultaneous use of an account? For example, an account with *Session-Timeout *2 hours is shared by two users. User1 login first and use the account for 1 hour, user2 then login (user1 still accessing the account), then user2 turns out two have *2 hours*, which is wrong and is because that user1 not yet send Acct-Session-Time to the server. If RADIUS is not suit for the scenario above, should I try other protocols such as DIAMETER and TACACS+? Thanks.