I have tried searching online with a broad variety of keywords, all of which turned up results for "shooting with both cameras on the same device".
Searching Video Production Stack Exchange also didn't help.
I am making a physics experiment and I will borrow iPhones for their high-quality graphics and built-in slow-motion. I will be recording two angles, so how can I get both iPhones to start filming at exactly the same time? Human error is not allowed so I can't get a helper and say "3, 2, 1, go!" so is there anything such as software, wires, or even a separate control from a third device that I can use?