I agree there should be only one tag. Which one? That will depend on which questions the tag is used for.
I suspect it would be most useful when discussing the canonicity of additional texts in the restorationist movements - if you're talking only about the standard Bible then the bible tag is sufficient. The questions would be along the lines of "What rationale do X have for considering Y to be the scriptures?" So I think I'd lean towards scriptures being the better tag, but not by much.
As it happens scriptures has been used more anyway. But probably both tags could do with a clean up, to remove them from questions where the scriptures aren't the focus, but are used just because a verse was referenced or requested. Just like the bible tag is used too much...
The tags, as currently used, are synonymous as near as I can tell. In my dialect of English, "scripture" is the correct term. The Bible is scripture. Mark is scripture. James and Jude are scripture. You can't count scripture. Although I suppose the Mormons might tell you there are four scriptures ...
The non-religious use of "scripture" for something reliable is an analogy to the religious use. I think that should never be used by Christians, at least on a site where it is likely to be confused with the real thing.
I suggest "scripture" as the primary term, and marking "scriptures" as a synonym. But only since I doubt if we can convince people not to use the latter.