"Double means showing both English and Chinese subs in 2 lines simultaneously."
And in his reply (English subtitles, 63:50 - 65:14), Dean Jansen said:
"figuring out exactly how ‑ what the expectation is and how something like that would work, we definitely just need to get a little bit more information and I think we can and should be able to figure something out." (English subtitles, 65:00 - 65:14)
Isn't playing only one subtitle track at a time a characteristic of CC subs in general, Dean?
There is a work around, though: have the 2 languages in each subtitle, separating them with a line break, now that Amara supports line breaks. See e.g. Jonathan Coulton - First of May - ASL Song. True, in this case, both lines are in English: one transcribes what is sung, the other what is signed. But you could just as well have one line in English and one in Chinese.
Things become a bit tricky when one language is written L->R and the other R->L: see the misplaced ♪'s in some subs of Khabar E Tahayyur E Ishq Sunn خبرِ تحیرِ عشق سن (in this case: real Urdu R->L and romanized Urdu L->R). Otherwise, separating languages with a line break in the same sub works fine.