You are correct! In SYNC mode, the radar acts as a single transducer, controlled by either the left or the right CDU, and displayed on the pilot's ND, copilots ND, and MFD with all the same parameters and modes. In SPLIT mode, the radar acts as if it were two separate transducers, controlled separately by the left and right CDU's. This would allow the pilot and copilot to each control the radar as they see fit for the situation, but also to display a different radar depiction on the MFD from the pilot's ND. As you said, this might be most useful for keeping an eye on long range weather on the MFD while looking at nearby terrain on the ND, or a similar combination.