MP is all but one parameter that would influence photo quality. It determines the maximum number of pixels in the image that the camera would take. It doesn't determine how good the sensor is, whether it's better taking low light, etc etc...

Whether the lense is good optically will contribute to photo quality. On a smartphone, the software package that processes the sensor output is also very important to the perceived quality of the photo.

There are also some phones with optical image stabilization now, which would help with counteracting against shaking.

With that out of the way, there are also different ways smartphone manufacturers made use of dual camera. In the past, there was a bit of a fad to have 2 of the same camera to take 3D pictures. That didn't take off.

iPhone 7+ went a different route, with a single wide angle, and a 2x telephoto, so it's possible to take some zoomed in shots without resorting to software zoom. That and using AI techniques to take portraits to mimic wide aperature DSLR shots.

So to determine what is "better", what kind of photography you want to take? If you like night shots, and don't care about portraits with the fake blur background, then you probably don't care about dual cameras.

So figure out what you want, and look at actual reviews comparing the photos instead of deducing quality from a number.

Edit: To respond to your intended use, having two cameras most likely won't make any difference compared to a single camera, unless the phone you pick has a telephoto vs wide angle camera, and you'd want the optical zoom capability.

Heat on a camera phone, when it's doing a lot of processing is unavoidable. A large phone with metal back may help with dissipate the heat, but recording long videos are not good for battery life.

Another alternative might be to find a used DSLR / mirrorless in good shape, and use that to record videos. It would give you a lot more options for your video production, at the expense of overall cost of owning a piece of hardware that only does video.