I developed a tool (https://github.com/smacke/ffsubsync) which can sync subtitles against each other (or even against an audio track), and this can be used in conjunction with other tools such as https://pypi.org/project/srt/ to combine multiple subtitle streams into a single stream. I've used this strategy to good effect to get both English and Chinese subtitles up at once.
For Netflix and YouTube I actually use a language learning chrome extension called Migaku, that has this feature - but if there don’t exist subs, can also sometimes pull it off with a chrome extension for dual subs - forget what it is called but I can download SRT files and load them up alongside a stream. Both are not reliable and require significant fiddling.
