The only time I've used two cameras to video something was a couple summers ago when I did a train video. I have no idea how to sync using a clapboard. I didn't have both cameras rolling at the same time either. I just edited the video based on the location of the train on the track making it seem like they were recording at the same time. Didn't spend a lot of time on it but it turned out OK.
I think you are looking for timecode synced with genlocks. At least some of the Sony cams support timecode, look for TC settings in your menu. Genlocks are external hardware devices to keep TC in sync between cameras. I am less clear on Sony support for this technology, but my understanding is there is at least some support for it. I am not a video guy, just know enough to point you to it and that is about it.
I found this introductory video, sounds about right from what little I know.