I had a two channel camera which functioned for over two and half years. It was nice having incidents in the same MP4 file, such clips of vehicles overtaking me on the paved shoulder and then cutting me off. In the VLC viewer, I could have two windows displayed showing incidents occur concurrently from the two cameras.
The rear camera died on the two channel system. I have kept the front camera mounted, but I have since purchased higher resolution single channel cameras for the front and back, giving me two cameras at the front and one at the back. I synchronised the time and date of my cameras and use the event button of the newest front camera. I then look at the time and date on videos from that camera to pinpoint the time I should look at clips from the rear camera. This works as long as I get to the rear camera before the clips are overwritten.
Some of the times if its an event where only the rear camera is necessary, and its not a significant event, like a distracted driver on his or her phone that is visible in the rear view mirror, I just take a mental note of the time or note where I am and look for the clip in the evening.