The iPhone X records 4K video to a memory card, using HEVC, H.264, MP4 compression formats, with frame rates of 24, 30 and 60fps available. If you want to record 4K video, then you'll need to use an external recorder connected to the HDMI port. The dual video recording option becomes available when recording at 24fps, so that you can record both to an internal memory card, and an external recorder at the same time.
