This paper addresses the design and control of autonomous mobile platforms manipulating and transporting a grasped object. Each platform is equipped with a manipulator arm. In order to hold an object in a stable grasp, the arms have to apply and maintain appropriate contact forces to ensure force closure. We describe an architecture and a real time control system that allow the coordinated control of multiple, heterogeneous, mobile manipulators. In this architecture, a lead robot plans, based on available sensory information, and follows a suitable trajectory. The other robots follow a desired formation with respect to the leader while maintaining a stable grasp. There are three main contributions in this paper. We present the real time control system and our approach to rapidly prototype the control code. We describe a novel, forklift-like arm and a control scheme that allows us to easily control the Cartesian stiffness or impedance. Finally, we describe the details of an experimental testbed and results that demonstrate the robustness of the control scheme.