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