In terms of ease, the Nextbase dual dash cam setup would be better because the devices have one cable linking them together. Otherwise you'll need to wire and power two different dash cams to the 12v power source, though this isn't that difficult as long as you have a dual port car charger - something like this (kit.honestjohn.co.uk/reviews/review-anker-24w-powe.../). Using a dual dash cam set up like the Nextbase 512GW and 512GWRC will provide good quality footage - as you'd expect from Nextbase - and it'll be easier to watch the footage together because they're downloaded onto one SD card. But it is arguably more expensive than getting two cheaper dash cams.

Another option would be something like the Z-Edge S3 dual dash cam. However, I recommend you watch some sample footage on youtube before buying to see what the rear dash cam footage is like. It's not uncommon for manufacturers to focus more on the front camera, so the rear camera often films in lower resolution and can be a bit rubbish. Lastly, if you are mounting a camera at the back of your car then use a sticky type mount rather than a suction cup because it might fall down when you slam the boot lid.