Actually the problem isn't that you can only have one link from a wikisource work from a wikidata item. We have separate wikidata items for each edition of a work (because these have different metadata) so multiple editions of the same work on a wikisource link to different wikidata items. 

This creates a different problem. Each language edition of a work is a different edition so it links to a different wikidata item which has sitelinks only to that translation of the work. This means you can't use sitelinks to link to translations of a work on other wikisources. 

Does this mean wikidata sitelinks are useless for wikisource?

filceolaire