In order to increase the role of machines in supporting more capabilities as regards a spoken dialogue system, we present in this paper a new problem incorporating multi-session in such a system. Instead of only handling single dialogue, such a system can take an intermediary role to communicate with many users in several discontinuous sessions for reaching a compromise between them. We describe here a new approach for modeling the multi-session and then we concentrate on the multi-session management of such a system dedicated for a complete service having several tasks.