Albion, and it's not just that you would have at least two authorities (if you accept Sola Scriptura), it's also that the first authority (the basis on which you accepted Jesus and the Bible) is a HIGHER AUTHORITY than Scripture because it DICTATED whether or not you accept Scripture as inspired.
And you can list more possibilities 3, 4, 5 and so on. In all cases, it means that Scripture is not the only FINAL authority in your life. 'Sola' Scriptura isn't Sola at all. THAT'S the charge of contradiction.
It all depends on how the word
authority is being used. God is the ultimate authority, we agree. But authority for
what? Determining our eternal fate? Deciding when the universe will come to its end? What? He is the final word on everything, right?
So when we are speaking of the final authority on essential dogma, that which we must believe as part of our faith...what is the authority? Or, related to that, what may any church rightly demand of its members in the way of doctrine?
The authority for THAT would be God. Also. Just as with everything else.
But how do we know what he is telling us in that regard?
It is in divine revelation, and just about every Christian church says that the Bible is the
word of God! What, in fact, could be a more reliable authority for determining essential doctrine than God's own revelation to us? And the term for using that revelation in preference to anything else we might think of is Sola Scriptura/the Bible Alone.
It cannot be either Reason OR Scripture, nor can it be Reason AND Scripture. That is because Reason is obviously not as high a faculty, not as authoritative as
Gods own word. However, it would be right to say that Reason is to be used by us in the process of understanding Scripture.