Using the same audio input on multiple devices, recording iOS video
There were no devices which were designed to filter matching audio into two iOS devices at the same time. The solution I went with in my comment is likely the best.

That is, Adobe Premiere and Final Cut Pro X both have a feature which will use audio to automatically sync up video tracks. I recorded high quality audio with an external microphone, and using that in conjunction with the audio built into each device, FCP:X was able to create a multi-camera clip with the three audio tracks and two video tracks synced up. I could then select each audio track and each video track independently.