PM's sent.

Regarding two subtitles at the same time--adding two subtitles to one file is certainly easy, but watching the movie with two sets of subtitles simultaneously will not be easy (I've never done it). Switching between the two--picking one or the other--is how it's typically done.

If you want to watch it in two alternate languages at the same time, you could use the Japanese or French dub, then use subtitles from the other language. The subtitles don't 100% match up with the dub tracks, though, since they're designed to accompany the English tracks, but they work decently.