In the old days the idea of running separate single cameras was most valid, but i dont feel it have much merit anymore.
New systems will alert you with beeping if there is a problem stopping it from recording, so a memory card failing mid drive, well you do at least get notified now, though the audio level of the alarms can be problematic ( too low ) and so easy to overhear, not least if you have the radio going or maybe a conversation.
Of course if the system fail outright, then you are out two cameras / all cameras, but in that situation if you had two single cameras and the front one fail you could move the rear camera to the front if it is the same brand / model and fit on the mount, or it / they use a universal mount type like suction cup.
No matter what kind of system you have, it is a good idea to have a spare memory card in the car, i have though the only times i have had any use for it is when i have forgotten a memory card for a camera in my apartment.
I have never had a memory card fail me mid drive, but i have had many memory cards go flaky on me forcing me to replace them to get the system stable again ( just done that with a samsung card in my K2S side system )
And i will also say when you have used dashcams for a while, at least i personally feel pretty naked if there are no camera, or i even worry if i have forgotten the memory card for one of several cameras on my windscreen.
Now i feel there are plenty of tried and tested dual cameras out there you can buy, and there are newer / smarter ones you can take your chance on CUZ it is a new brand / model, of if smart features for some reason are important to you.
There are even older dual systems, but thy then lag things like alarms for mad memory cards ASO so it might sit there silent and appear to record just fine, but it just are not doing that.
ATM i will at the moment advise against any smart camera, it often seem people have problems with the smart part of the cameras, but the basic recording function of the camera might still be working.