Like camcorders or actioncams, is there any way to see what is being recorded on another cellphone?
This almost certainly isn't the answer you were looking for, but if not it may be useful for others.

If you have an Apple Watch, you can actually use that to view what is being captured within the iPhone's camera frame, both for photos and video. You can also use the Apple Watch as a basic remote to control the iPhone's camera (take photos, start/stop recording, etc). Obviously, this only works for the iPhone the Apple Watch is paired to, not for other iPhones or devices.