On your iOS device, launch Camera App. Select the desired recording setting (eg. Video).

THEN plug in your external Audio Interface.

Record. It will take in whatever is on channels 1 and 2 and record with your video stream.

NOTE: If you either close the Camera App, power down the iOS device, iOS device goes to sleep or you disconnect your interface - you will need to repeat the above procedure to capture again.

Other issue, these is NO INDICATION you are recording audio from the external interface - so you may want to test the procedure a few times to make sure you know it works for you so you can reproduce exactly in a real situation.