Here is a very simplified example of split and stitch encoding with ffmpeg. Such as setup could be used for spreading encoding across a cluster for parallel encoding or large files or just really fast encoding. It has some limitations in that it needs more than optimal keyframes for best size/quality combinations but on the plus size it would be compatible with segmented delivery of files. Note that the MPEG Transport Stream format has been used as it is the most compatible for stitching back together.
Next I will do some more investigation into the GOP structure generated by ffmpeg in this scenario.
In the first section a source file is broken into 3 x 30s parts (note that this is not the whole clip and is just for demo purpose) and is transcoded into a h264 (libx264) transport stream.
The second step is the stitching back together via a simple concat filter, as the files are a transport stream and encoded with the same settings this works well.
ffmpeg -y -i anchorman2-trailer.mp4 -ss 00:00:00.000 -t 30 -c:v libx264 -s 640x360 -b:v 1000k part1.ts ffmpeg -y -i anchorman2-trailer.mp4 -ss 00:00:30.000 -t 30 -c:v libx264 -s 640x360 -b:v 1000k part2.ts ffmpeg -y -i anchorman2-trailer.mp4 -ss 00:01:00.000 -t 30 -c:v libx264 -s 640x360 -b:v 1000k part3.ts
ffmpeg -y -i concat:part1.ts\|part2.ts\|part3.ts -c copy concat.ts