This feature will be useful for bilingual works that have one language in odd pages and a second language in the even pages, since Wikisource wikis are language based.
I've just found 89 librettos in this scennario . I don't have plans to work on those in the next months, but this feature will be a great addition if more works in this page configuration starts to appear in the web.
Taking for example , the given syntax can be, for the Portuguese language index:
For the Italian language: