This paper describes the issues raised by the evaluation of multimodal interfaces in the field of command and control workstations. Design, specification, verification and certification issues for such Man-Machine Interfaces (MMIs) have been already identified as critical activities. This paper focuses on the issues raised by evaluation of their usability evaluation. We first present a formalism (Interactive Cooperative Objects) and its related case tool (PetShop) for the specification of such MMIs and then show how the models built can support the usability evaluation phase. As a case study we present a multimodal interaction for 3D navigation in a 3D satellite model.