How to *integrate* every two frames of an existing video into one in order to compensate for low illumination? Some mobile phones are able to compensate for low illumination conditions by integrating every two or more frames into one. The result is a video with variable frame rate, where each image has its particular timestamp. This kind of footage shows correctly illuminated subjects, but you notice the drop in frame rate because the movement is not fluid and also the images become noisier (because you are adding too the noise produced by the electronics). Anyway it seems a technological miracle for me, taking into account that the entrance pupil of such devices may not be larger than 1 mm in diameter.

But let's say you forced the phone to record at a fixed frame rate (e.g. 1280x720p30 with OpenCamera in Android). It should be possible to convert for instance 30p to 15p by adding the values stored in the corresponding pixels of each two frames into one. I know this process is possible because it is usual in other contexts, e.g. dealing with astronomical images.

My tools are ffmpeg and Blender. I would be thankful if someone can restrict the answer to these tools, though other solutions are also welcome because they might put me in the right track to look for suitable alternatives.
As you note, just adding (integrating) each pair of adjacent frames to make one new frame decreases the frame rate.

But if all you want to do is integrate adjacent frames, consider duplicating the video into a separate timeline, shifted by exactly one frame. Integrating these two streams would give you a new video with the same frame rate, but shorter by one frame overall.

All the rest of the noted issues might surface, but at least the frame rate wouldn't change.