Certainly when the film and the viewing screen are the same size and distance from the lens and the viewing and taking lens are the same focal length it is quite easy.
Originally Posted by Steve Smith
But in my example the film would be 6cmx6cm but the sensor would probably be 2.5cmx2.5cm, requiring two different focal lengths and a progressive linkage to keep the focusing synchronous. But that would be the only 'hard' part.
The upper chamber, with its smaller sensor closer to the lens, would have plenty of room for batteries and electronics.
It would be possible to take a single simultaneous shot on both media.