Now you are switching the subject. The question is not whether Bible or Koran can "demonstrate the existence of any God" to a skeptic, but if content of Koran is in sufficient agreement with the Bible to consider God of Abraham, Isaac and Jacob as God of Muhammad.
Go to the library and do your homework, son.
How so? If neither document demonstrates a deity in the first place, why should I use either document as a proof of anything regarding this matter? I mean, in the big scheme of things it's a moot point. No deity demonstrated in the Bible. No deity demonstrated in the Koran. Useless to argue which deity is real and which not (or whether they're both the same) when neither have been demonstrated in the first place.