One soul root

I learned that when we are born, woman and man are together as one, we are then separated until we are married and find our beshert. if so, how come polygamy was acceptable prior to the ruling of r’ gershon? how could it be that king Shlomo had a several wives? how can we explain the concept of beshert?

As you know, we are not 'born' with our wives. You refer in your question to the talmudic teaching that before we were born it was determined who would be married to whom (Sotah 2a; Sanhedrin 22a). This teaching, according to the talmud, refers to one's first marriage. King Solomon was rebuked (Sanhedrin 21b) for marrying more wives than a king is allowed by the Torah (eighteen-see Rambam Sefer HaMitzvot prohibitions 364, Laws of Kings 3,2). Jacob's marriage to more than one wife reflected his complex spiritual identity characterized by more than one aspect.