Using old Android phones as multi channel dash cam Now a days the Android phones are having quite a good computing power and good optics. I have heard and used my daily driver Android phone as my dash cam.

I have also heard about dedicated dual channel dash cams which can record from two cameras (ideally one placed to record what is happening at car's front and another on rear and combine those to into one single video).

With few spare Android phones lying around I thought of building myself a dual channel dash cam purely using old Android phones.

The challenges I have:


The separate feeds from both phones should be merged to show side by side.
Can the phones be kept on and recording 24x7 even when the car is parked?


Advantages:


It also acts as a CCTV for my garage area recording all activities.
Android phones take considerable time to start up.  And in this mode the recording will be available continuously. Also the dash cam app need not be auto started during boot.

Disadvantages:


Not sure about its impact on battery life.
Not sure about its impact on phone's and camera's life.


Overlaying the latitude, longitude, speed, date and time information on video.
Retain videos for later use on tap of the screen.  Usually these kind of dash cam apps will run in loop erasing the previously recorded video so that it will not run out space. On tapping the screen during some moments to preserve it copies the current recording block as a new file so that it is available for future use. In front only dash cam scenario this works well.  How can i do this in front and back dash cam so that the rear dash cam also preserves on tapping the front cam?


Question:
Is there any solution that addresses this need? If not any partial solutions to the challenges?

I know this sounds like a good question for a project on Instructables, but this involves considerable Android software and power user's experience.  In case if this sounds a bit of out of topic, please let me know in comments.
Most of your requirements can be met by either a custom software or some apps on the Play Store.

May be not merging video. You will need to upload chunks of video periodically, be it hourly, daily or weekly, to some central location that knows about all devices. (Some server in the cloud? your home PC? May be you can set the save location of the apps to a OneDrive/Google Drive folder)

I don't think there's any harm to the camera on devices on atleast most devices. I've seen people use droids for home surveillance for a while.

Although you'll have to make sure the device isn't placed in direct sunlight and not exposed to extreme summer heat. Most devices can shut down automatically when the heat is more than it can handle, though it is mostly to prevent heat damage from an overloaded/misbehaving CPU, not the heat from the environment.

Battery Life

Main issue would be the battery life. Even when simply recording by using a charger from the car (usually 1A. May be 2.4A if you're really lucky to get one of those charger adapters.)

Your camera won't last more than a hour or two under it's own power. (Even the latest devices can only last 5-6 hours with screen on) You can may be attach a battery pack which uses solar panels to charge.

I don't know the effect on the battery when its connected to a constant power supply. Many new units can handle a good amount of abuse, but older ones may have issues. Although this isn't an issue for you as long as you have a way to constantly power the device.

Things may become a bit simpler if you only need to record when the car is running. This will reduce the battery issues significantly.

Due to the openness of the question, you may have better luck posting this on a discussion group/forum or something like Reddit.