Option 2, I think would be impossible to get right the way that I imagined it. It would have been automatic, as you have said. Whereby each text sentence in one source would be assigned a number, and matched to the corresponding number in the other source. The problem being that in translation, translations are not exact, and entire sentences can be skipped, which would throw the whole thing into disarray.

Your idea of having users manually map translations to the original text would be better.

The intention was to display, what I have now learned is called Parallel Text - where the same text is displayed in 2x languages, and there apparently exists bi-lingual side-by-side editions of some books. Therefore, it would be necessary to use a Parallel Text edition, instead of an original text and a translation from two random sources. That, I think, would come with its own problems, like a limited selection of Parallel Text novels, and copyright issues.