This idea being projected that the Bible was polytheistic is in error, it is a limited perspective of global religion - the ancient Hebrews first had a theology similar to the Dharma, El Elyon is the same as Brahman, and Bhagavan Brahma means 'Lord who makes Become' in Sanskrit, Yah-Havah means the same thing from Hebrew.
It is correct that after Babylon the language was changed on purpose to match a form of henotheism, where they rejected the previous theology as they felt it was similar to the Canaanites/Babylonians system.
El and Elohim are not the same thing as stated in Isaiah 46:9, where it tells us to 'Remember the former things of old', using the same language used in Deuteronomy 32:7-9, where El Elyon separated the nations among the Elohim/Children of God.
It is interesting when we question that things get corrupted after Babylon, and then examine the texts before then, where in these verses:
David included YHVH & the God Most High in all these Psalms:
Therefore the problem wasn't that there are two distinct entities like in other Middle Eastern religious systems, it is more the comprehension of what these are seen as in the ancient Hebraic understanding.
The Bible is quite clear on these concepts:
El Elyon is the Father, and is the Source of reality.
YHVH is the Creator Divine Being, and is seen as the Arm of the Lord interacting with reality.
The Messiah is the Hand of God interacting in person with this world.
The Divine Council is reiterated in the book of Revelation, where there are 24 Elders surrounding the Throne of God in many religions (Buddhism, Jainism, Hinduism, etc), and the Quran was also trying to repair Middle Eastern Divine Council theology (38:69, 37:8) of not worshiping the individual Lords, yet to worship the God Most High alone (Ala Ilah = El Elyon).