  Humans show specialized strategies for efficient collaboration. Transferring
similar strategies to humanoid robots can improve their capability to interact
with other agents, leading the way to complex collaborative scenarios with
multiple agents acting on a shared environment. In this paper we present a
control framework for robot-robot collaborative lifting. The proposed shared
controller takes into account the joint action of both the robots thanks to a
centralized controller that communicates with them, and solves the whole-system
optimization. Efficient collaboration is ensured by taking into account the
ergonomic requirements of the robots through the optimization of posture and
contact forces. The framework is validated in an experimental scenario with two
iCub humanoid robots performing different payload lifting sequences.
