I have spent many hours searching, but can't find a dual camera front & rear dashcam system - where both cameras are inside the vehicle.
There are hundreds where you mount the rear camera outside the vehicle, to help with parking. I don't want that. I woudl really prefer a syste where both cameras are mounted on the rear-view mirror.
Would it be on or off-topic to ask for a recommendation?
On or off topic? - It's off topic, to be "hardware", according to this site's definition, it needs to be connected to a computer at least most of the time to operate.
On almost all SE sites you can visit the (❔) in the upper right corner, choose "Help Center", and view the FAQ titled: "What topics can I ask about here?" - occasionally that hasn't been edited and contains a generic template, in that case there's often a Meta discussion that reveals such info.
A search for "dual camera front & rear dashcam system mirror" turns up plenty of results: "Falcon Zero F360 HD DVR Dual Dash Cam", "DuoView - 2.7 Inch Car Rear View Mirror with Dual Dash Cameras", etc.
