The two tags tag-synonyms and synonym-request have almost identical tag wikis and seem to be equivalent. Should not one become synonym of the other then? I found this question: Tag synonyms on [Meta]: which asked for it but there was no specific comment/reaction to it.
I don't think they are synonymous:
All tag synonym questions need not be synonym requests. (And I don't think tag-synonyms would necessarily be appropriate for synonym requests.)