There are a few devices which included an option to shoot both simultaneously, putting a small selfie of the user in the corner of the main image.
This was touted a year or so ago, but I've not heard anything much about it since. Possibly it turned out that very few people were actually interested in it and the feature was quietly dropped, or at least not talked about (to me it looked like the sort of gimmick someone would use twice, decide it just spoiled the photos, and never use again).
Was only for a few devices, not sure whether anyone ever developed an app for it more generally.