Gen.1:3 "Let there be light." The beginning of the Second-Earth-Age. People are born of women during this time to choose their Father: which One they will follow, serve, and love. This means that their are Two Fathers to chose from. Two is a symbol of choice.
Beginning in Gen. - Two lights. Two Trees in the Garden: Life = Our LORD. Knowledge = Satan.
Noah's Ark: Two of every sort. Rebekah and Isaac: Two nations in thy womb. Elijah challenge[1K.18:21 How long are you going to halt[jump/leap] between Two opinions.
Immanuel-Christ-Jesus[Matt.6:24 "No man/woman can serve Two masters..."
T-W-O is throughout the Word. Significance, GOD's math. Marriage. From the beginning of the Second-Earth-Age the objective was to be "fruitful and multiply: replenish the earth." Not sure where multi-wives came from, but the Word is clear from Gen.2:21-24 = "leave and cleave." "The Two are one flesh." The LORD repeats HIMSELF again in Matt.19:5,6; from Gen.2:21-24.
Inside the Kingdom of Heaven on - Earth: an "open marriage" does not exist. An Open Marriage" - is No marriage. One can only have an "Open Marriage" with Satan.