I would strongly recommend recording the dual streams. If you have the substream set to a low bitrate (mine or 256), you will be glad you did if you ever try to do a playback of a cascade tile of watching all your cameras playback at the same time. If you have all your cameras showing playback at the same time with the mainstream, many systems will struggle playing back unless you have few cameras or a screamer system. If you then select playback on one camera, it will show the mainstream then.
But the advantage to running substream helps drop CPU during live recording, but the recording substream helps on playback and watching many cameras at once. Most do not realize that is the benefit. But many also have no need to watch multiple cameras playback at the same time so then it doesn't matter.