To turn on double subtitles, please turn on Learn Mode in the video player settings.
If Learn Mode is grayed out and cannot be switched on, it is usually because the video does not have sufficient subtitles in the language of your choice or subtitles in the original language (Chinese, Japanese or Korean).
You can learn how to use Learn Mode by referring to this FAQ:How to use Learn Mode?
Also, here is a collection of dramas which have Learn Mode fully enabled:
Please also note that Learn Mode is currently only available on desktop web browsers.