So obviously the part of the argument that is difficult is the idea that more than one person could be included in the identity of God, especially from a Hellenistic Jewish perspective. I think that part is so difficult that the argument is being misunderstood because the assumption that this is impossible is so strong.
However, there is evidence that jews in the couple centuries prior to Paul sometimes wrote about God in this kind of way. Specifically in the wisdom literature that was created at this time, and its personification of "wisdom". Consider the
Wisdom of Solomon, composed by hellenistic jews somewhere in the 2nd or 1st centure B.C.E
"For wisdom is a kindly spirit and will not free a blasphemer from the guilt of his words; because God is witness of his inmost feelings, and a true observer of his heart, and a hearer of his tongue. (Wis 1:6)
Wisdom is radiant and unfading, and she is easily discerned by those who love her, and is found by those who seek her. She hastens to make herself known to those who desire her. He who rises early to seek her will have no difficulty, for he will find her sitting at his gates. To fix one's thought on her is perfect understanding, and he who is vigilant on her account will soon be free from care, because she goes about seeking those worthy of her, and she graciously appears to them in their paths, and meets them in every thought.
The beginning of wisdom is the most sincere desire for instruction, and concern for instruction is love of her, and love of her is the keeping of her laws, and giving heed to her laws is assurance of immortality, and immortality brings one near to God; so the desire for wisdom leads to a kingdom. (Wis 6:12-20)
Or the
Wisdom of Sirach, in the same period:
"All wisdom comes from the Lord and is with him for ever. The sand of the sea, the drops of rain, and the days of eternity -- who can count them? The height of heaven, the breadth of the earth, the abyss, and wisdom -- who can search them out? Wisdom was created before all things, and prudent understanding from eternity. The root of wisdom -- to whom has it been revealed? Her clever devices -- who knows them? There is One who is wise, greatly to be feared, sitting upon his throne. The Lord himself created wisdom; he saw her and apportioned her, he poured her out upon all his works. She dwells with all flesh according to his gift, and he supplied her to those who love him. (Sir 1:1-8)
Wisdom exalts her sons and gives help to those who seek her. Whoever loves her loves life, and those who seek her early will be filled with joy. Whoever holds her fast will obtain glory, and the Lord will bless the place she enters. Those who serve her will minister to the Holy One; the Lord loves those who love her. He who obeys her will judge the nations, and whoever gives heed to her will dwell secure. If he has faith in her he will obtain her; and his descendants will remain in possession of her. For at first she will walk with him on tortuous paths, she will bring fear and cowardice upon him, and will torment him by her discipline until she trusts him, and she will test him with her ordinances. Then she will come straight back to him and gladden him, and will reveal her secrets to him. If he goes astray she will forsake him, and hand him over to his ruin. (Sir 4:11-19)
These are not "canonical" texts either in Christianity (deutero-canonical-ish in catholicism and orthodoxy) or in Judaism, but they do illustrate very well the way some hellenistic jews in this time period could speak about God, and the comparison to the way Paul talks about Jesus is clear, I think. It doesn't seem to make sense to entirely separate this character of "Wisdom" from God, given that she is explicitly an agent of God's justice, grants perfect understanding, that the laws (which must be the laws of Moses) are "her" laws which grant immortality, and etc.
Obviously there is a difference between the personification of an attribute like wisdom in a literary sense and an actual person, but even so I think the usage, and the similar usage of the "Spirit of the Lord" demonstrates that there isn't this automatic inability for jews of the period to reflect on the nature of the one God in a similar way.