A boon for the mobile content creator: The FiLMiC Pro app has a clever multi-cam feature that allows you to exploit the multiple cameras on the iPhone XS/XR and 11. Now you can choose more than one camera and record video from them simultaneously--including from opposite faces of the camera, meaning you can record two-person face-to-face interviews, without having to carry a second camera.
If that's all the app did, you'd still have to overlay those two videos in post-produciton. But FiLMiC Pro goes the extra mile by allowing you to choose whether the videos should be saved discretely, or split-screen, or with picture-in-picture. (And yes, you can select focus and exposure settings separately for each camera.)
Because operating all of these things could become quite confusing, I'm most impressed at how thoroughly the app's designers have thought the UI through. This is best witnessed by watching the tutorial:
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The app will set you back 15 bucks and it's available here.