Even when using a stereo camera, there is no such thing as a "stereo camera info" message. Instead, the left and right cameras publish each publish their respective camera info. If you are writing a stereo camera driver node, then you should use a separate CameraInfoManager for each half. If you want to do your own stereo processing, then you should use the TimeSynchronizer class in the message_filters package to synchronize the cameras' image and camera_info topics.