I don't think this is actually true. I have two dual dashcams, a mini 0906 and a Viofo A129 and they both appear to be better quality and more reliable than their single channel predecessors.
In the case of the 0906 it does not suffer the well known battery problems of the 0806, because it is using the new technology of super capacitors.
It also doesn't suffer from melted plastic mounts, the lesson was learned and the mount now uses high temperature plastics.
Both cameras are using the next generation of processor which is improved, less power hungry, lower heat production.
Both have a new generation of lenses compared to their predecessor for improved image quality.
A lot of lessons from the previous generation have been learned and the cameras improved.
The only argument I can see for two being more reliable is that if one was to fail, or it's memory card or it's power supply, then the other single camera would keep going, but when I had single cameras I rarely checked the memory card in the rear camera, I think I'm a lot more likely to notice a problem with just the one camera to check and then get it sorted quickly, and of course with only half the cameras there is only half as much to go wrong, with the exception of an extra sensor.