What is the “two powers of heaven” idea and is it scriptural?

This idea was mainly promoted by a 1977 book ‘Two Powers in Heaven: Early Rabbinic Reports about Christianity and Gnosticism’ by the American Jewish scholar of religion Alan F. Segal. In his book he proposed that some early references in rabbinic texts were directed against beliefs of Jewish Christians and Gnostics.

Segal’s thesis was heavily based on Jewish answers to passages claimed as pre-existence passages by Trinitarians and Arians, but seen only as prophecy passages by those believing that Jesus did not exist until his conception and birth as taught by the New Testament.

 13 “I saw in the night visions, and behold, with the clouds of heaven there came one like a son of man, and he came to the Ancient of Days and was presented before him. 14 And to him was given dominion and glory and a kingdom, that all peoples, nations, and languages should serve him; his dominion is an everlasting dominion, which shall not pass away, and his kingdom one that shall not be destroyed. (Daniel 7:13-14 ESV)

So basically, no; the two powers of heaven idea is shown by the key text Daniel 7:13-14 to not be scriptural. Christ did not exist before birth, and Jews did not believe that the Messiah preexisted in heaven.

Footnotes
1. new edition publishers blurb 2012 reads “In his now classic Two Powers in Heaven, Alan Segal examines rabbinic evidence about early manifestations of the “two powers” heresy within Judaism. Segal sheds light upon the development of and relationships among early Christianity, Gnosticism, and Merkabah mysticism and demonstrates that belief in the “two powers in heaven” was widespread by the first century, and may have been a catalyst for the Jewish rejection of early Christianity. An important addition to New Testament and Gnostic scholarship by this much revered scholar, Segal’s Two Powers in Heaven is made available once again for a new generation.”