To truly support this, you'd need to run both cameras simultaneously, which is not currently supported. The best you can do is use an AVCaptureSession with a VideoDataOutput and record it to AVAssetWriter. When you want to change cameras,
// ENSURE THAT BOTH CAMERAS ARE CONFIGURED TO GIVE YOU THE SAME RESOLUTION!!!
You'll notice a brief glitch in the recording where cameras were switched (audio will glitch too), but you'll at least be able to continue recording to the same movie file (AVAssetWriter stays open across the swap).
The main issue is AVCaptureAudioDataOutput hangs while you switch cameras which makes it useless. I have filed a bug many years back and it still remains open.