[kadoban]

Since srt files are just a list of timestamps and things to show, I bet you could create a simple tool that just interleaves the two language subtitle files. Not sure how that would look.

Just based on the options it allows you to choose, https://subtitletools.com/merge-subtitles-online seems like a good bet to me. I'd try playing it in vlc, it generally supports anything a little odd that other players may not like.