This paper describes a new vision-based control method to drive a set of robots moving on the ground plane to a desired formation. As the main contribution, we propose to use multiple camera-equipped unmanned aerial vehicles (UAVs) as control units. Each camera views, and is used to control, a subset of the ground team. Thus, the method is partially distributed, combining the simplicity of centralized schemes with the scalability and robustness of distributed strategies. Relying on a homography computed for each UAV-mounted camera, our approach is purely image-based and has low computational cost. In the control strategy we propose, if a robot is seen by multiple cameras, it computes its motion by combining the commands it receives. Then, if the intersections between the sets of robots viewed by the different cameras satisfy certain conditions, we formally guarantee the stabilization of the formation, considering unicycle robots. We also propose a distributed algorithm to control the camera motions that preserves these required overlaps, using communications. The effectiveness of the presented control scheme is illustrated via simulations and experiments with real robots.