The original note, as written at the time
An app that allows one smartphone to access the camera app on the other smartphone through a wireless connection and take pictures secretly.
Ongoing development
An app that lets one phone reach the camera on another phone over a wireless link, and take pictures without the second person knowing.
I will not pretend this is a neutral piece of engineering. It is the exact capability that produces the worst stories of the last decade, and a manufacturer that shipped it openly would be shipping a stalker tool with a settings menu.
So the honest version is the one where the second phone knows. Not a notification after the fact — a visible, persistent indicator that the camera is live, a shutter that the person holding the device can hear, and an off switch that cannot be overridden by the remote end. That is the whole difference between a tool and a weapon, and it costs almost nothing to build.
With that constraint, the useful cases are real and they are all about a camera that is already in the right place. Propping a phone to watch a pot, a sleeping child, a doorway, a machine that is making a noise it should not be making. A second phone as the viewer is a free monitor, and the hardware for it is already in two pockets.
The interesting design question is the consent model. Two phones, one owner, is the easy case. Two phones, two owners, needs something better than a checkbox: a lease that expires in hours rather than days, a session that both parties can end, and a record both can read.
Everything here is already possible. That is why the rules matter more than the feature.