Interestingly, I wrote about parallel texts here:
http://www.albertopettarin.it/rs.html
In short: I think that this kind of functions should be put in the reading systems, not in fancy CSS/JS gimmicks. The eBook should contain only those metadata that make the app "recognize" that parallel display (or other "advanced" rendition) is needed.
(A
picture is worth 1,000 words, right?)