I think they are more concerned about multiple threads of the same client trying to access the same SFSB instance. However you could probably serialize the handle, send it to other client(s) over the network - unless a business method is currently in progress the container would have no idea whether the original client is going to use the SFSB instance again.
EJB 2.0 Specofication p.76
7.5.6 Serializing session bean methods
...
Clients are not allowed to make concurrent calls to a stateful session object. If a client-invoked business method is in progress on an instance when another client-invoked call, from the same or different client, arrives at the same instance of a stateful session bean class, the container may throw the java.rmi.RemoteException to the second client[4], if the client is a remote client, or the javax.ejb.EJBException, if the client is a local client. This restriction does not apply to a stateless session bean because the container routes each request to a different instance of the session bean class.