In business or in industry, some entities are in collaboration with each other when they work together with or without common objectives. In this paper, we are interested in this collaboration relationship in the context of aeronautics. More precisely, we focus on a use case in which two actors' objectives are respectively to design an aircraft and to design the assembly line for this aircraft. Following some previous work on coopetition, we analyse the dependency relationship between these actors and propose i * models. In order to solve dependency cycle issues, we introduce a third actor that is in charge of realising trade-offs between the two designs. Finally, we show how existing methodology could be applied for supporting this trade-off activity.