A multimodal presentation planning mechanism must take into consideration the structure of the discourse and the restrictions imposed by partial plans generated in the early stages of the planning process. The latter requirement demands that the planning mechanism be able to transfer plan constraints from one level of planning to the next and to modify partial plans locally at each level. In this paper, we introduce a multi-agent planning mechanism based on the blackboard architecture that satisfies these requirements, and describe the facilities used by agents to communicate with each other in this architecture. In addition, we discuss the planning processes used by the agents in our prototype system.