Says Who: Since we don’t find in the Torah such a concept that one may have two separately recognized names, therefore the proof lies on the one who is being “mechadesh” that there does exist such status in halacha. “Hamotzei M’chaveiro alav Haraya”.
I don’t see why we should need proof to imply that two worded names are recognized as one long name in halacha, since this is consistent with the Torah concept of one name per one person; and that which the names are spelled separately is insignificant in halacha and can hardly be counted as proof.