Ciao mcgyver83, welcome to the forum.
I like dual channel, and also recommend it most often, but i also have to admit that 99% of the footage i have pulled from my dashcams are from the front camera, though mind you the 2 times i could potentially needed a rear camera, well first time it did not work ( insufficient power source ) and the 2 time the parking guard had timed out, both times my car got hit in the back.
Fortunately the first time the guy hitting us took blame just fine, 2 time the woman hitting me left papers then 30 minutes later removed them, but fortunately little old lady seeing it all took down her plate so no cost to me in either case.
Needless to say i now do not use 1 hour timer for parking guard i use 3 hour timer.
If money are tight getting a A119 mini 2 as mentioned is fine, and you can reuse old system in the back, there is a slight downside of the 2 cameras footage then are not in sync due to different boot times, but that should not matter in a dispute i would think.
It is also a bit annoying having to go to 2 places for memory cards, but if you do not share footage often like i did in the old days, it is just fine.
Your A129 will of course be a little bigger on the rear window VS a remote rear camera, but if that do not bother you, it should be fine to use there, be sure to use a good power source, not a long cheap USB cable like i did and so missed me getting rear ended.
Some cars have a 12 V plug in the back, or you can install one there yourself, i did that back in the day when i realized my mistake,