Two single cams can do the job. For top cam performance it's the best way to go. It can also be the cheapest way according to the choice of cams.
Advantages: Better specs, wider choice of form factor, redundancy in case one cam fails, no length limitations on cabling to the rear cam.
Disadvantages: Card access to rear cam may be tough, you may have to add 12V wiring to power the rear cam, two systems to monitor for proper operation.
2-channel cams are usually easier to install and manage, and the better ones offer at least mid-level performance on both cams which is good enough for most people. Processor design and heat are the main limitations with these right now, and until there is advancement in those areas dual-channel cams will lag in performance. Dual-remote cams help with the heat problem but the main limitation is the processor so their advantage is a mostly in a more discreet install.
Wifi functions with dashcams are slower and more limited than most people expect, and issues with apps are somewhat commonplace. IMHO the best parking mode is constant low-bitrate recording but that can leave you having to search through a lot of video to find an incident if it's not tagged by a g-sensor or motion-detect function, and you'd need to discover any damage before loop recording overwrites the files you need. Buffered parking modes can make discovery and management easier but they depend on the g-sensor or motion-detect activating properly without too many false activations, and few cams do that well.
In my opinion, the A129Duo or the SG9663DC are currently the best dual-cam choices with the K2S providing an interesting dual-remote option, and the A129Pro can't be beat for a single cam. Rear single cam can be a B1W, F1, or Mobius Maxi for their small form-factors.
Phil
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