Segal's Persepective and its Reception
That book by Segal was one of the first to advance that argument, and has been fairly influential since then (e.g. Heiser, 2004; Boyarin, 2001. McGrath and Truex (2004) cite six other works influenced by Segel's book). Certainly there is a case to be made that some Jews (not all) in the late second temple period were binitarian in some sense; for instance Boyarin in Gospel of the Memra: Jewish Binitarianism and the Prologue to John, (2002).
An excerpt (p. 254):
"The Memra has a place above the angels as that agent of the Deity who
sustains the course of nature and personifies the Law." [Edwards 1995,
Justin's Logos and the Word of God, p. 263.] This position
has been well established among historians of Christianity since the
late nineteenth century. Alfred Edersheim saw the Memra as referring
to God's self-revelation. As Robert Hayward says of Edersheim: "He
also made a distinction between God and the Memra. Noting that
Rabbinic theology has not preserved for us the doctrine of distinct
persons in the Godhead, he remarks: 'And yet, if words have any
meaning, the Memra is a hypostasis.'" [Robert Hayward, 1981
Divine Name and Presence: The Memra p. 3] With this comment,
Edersheim is clearly implying the existence of non-rabbinic forms of
Judaism that were extant and vital within the rabbinic period
alongside the rabbinic religion itself. Although the official rabbinic
theology suppressed all talk of the Memra or Logos by naming it the
heresy of "Two Powers in Heaven," both before the Rabbis and
contemporaneously with them there was a multitude of Jews, in both
Palestine and the Diaspora, who held onto this version of monotheistic
theology. If we accept Edersheim's view, the Memra is related to the
Logos of Logos theology in its various Christian manifestations.
Emphasis mine. Also p. 249:
It becomes apparent, therefore, that for one branch of pre-Christian Judaism, at least,
there was nothing strange about a doctrine of a deuteros theos, and nothing in that
doctrine that precluded monotheism.
On p. 257 Boyarin quotes the Palestinian Targum for the Genesis 19:24 verse you cite (from Klein, 1980):
"And the Memra of H' rained down on Sodom and Gomorrah...
The idea is that in this theology, the Memra of God (Word of God) fulfils the physical, personified actions, while God himself remains invisible and totally transcendent from the creation. The way this theology could naturally develop (or be fulfilled, if you like) with a Christian incarnation should be clear.
Boyarin's article goes into quite extensive detail presenting the case, with relevant material from Philo, Justin Martyr, Targumic sources, as well as other Jewish prayers and texts.
But were Jews Binitarian?
Perhaps, but it's not really something that can be answered definitively, and is likely to remain controversial. Arguments can be made (like that of Segal, etc.) that some Jews were, but Jewish thought was not monolithic. As mentioned in the except from Boyarin, the rabbis opposed this line of thought officially.
Boyarin, D. 2001. Gospel of the Memra: Jewish Binitarianism and the Prologue to John. Harvard Theological Review 94:3 July, p. 243-284.
Heiser, M. S., 2004. The Divine Council in Late Canonical and Non-canonical Second Temple Jewish Literature. Doctoral Thesis, University of Wisconsin-Madison.
Klein, M. L., 1980. The Fragment-Targums of the Pentateuch: According to Their Extant Sources. Vol. 1, p. 45-46.
McGrath, J. F., Truex, J., 2004. 'Two Powers' and Early Jewish and Christian Monotheism. Journal of Biblical Studies 4/1 January. p. 43.