In this paper, we propose a control scheme that allows a humanoid robot to perform a transportation task jointly with a human partner. From the study of how human dyads achieve such a task, we have developed a control law for physical interaction that unifies standalone and collaborative (leader and follower) modes for trajectory-based tasks. We present it in the case of a linear impedance controller but it can be generalized to more complex impedances. Desired trajectories are decomposed into sequences of elementary motion primitives. We implemented this model with a Finite State Machine associated with a reactive pattern generator. First experiments conducted on a real HRP-2 humanoid robot assess the overall approach.