服务端处理+新版 FFMPEG hstack
https://ffmpeg.org/ffmpeg-filters.html#hstack
Stack input videos horizontally.
All streams must be of same pixel format and of same height.
Note that this filter is faster than using overlay and pad filter to create same output.

```
ffmpeg -i a.mp4 -i b.mp4 -filter_complex vstack output.mp4
```