While each "frame" might still be read sequentially by the digital server, they must be processed twice as fast. A 24fps 3D picture needs to process the content at 48fps in order to be able to get the images to to projector at the same time. So, rather than think of it as left/right, think of it as single stream of data which gets processed into separate channels. The stream can't exceed 250mb/s (500mb/s for newer systems) and the frame rate or number of channels within it isn't really relevant.
