Where is it found?
How is it that the religion that came up with the soul mate idea allows polygamy?soul mate is not a Christian idea IMO.
It is a Jewish idea though.
Genesis Rabbah 68 declares that heaven appoints a man's wife. Then in the Babylonian Talmud, in tractate Sotah 2a, we find the following: Rabbah b. Bar Hanah said in the name of R. Johanan: It is as difficult to pair them as was
the division of the Red Sea; as it is said: God
setteth the solitary in families: He bringeth
out the prisoners into prosperity! But it is
not so; for Rab Judah has said in the name of
Rab: Forty days before the creation of a
child, a Bath Kol (a heavenly voice proclaiming the judgment or will of God) issues forth and proclaims,
The daughter of A is for B; the house of C is
for D; the field of E is for F! — There is no
contradiction, the latter dictum referring to a
first marriage and the former to a second
marriage. (Taken from the Soncino Babylonian Talmud)
There are also other tractates from the Babylonian Talmud (I'm not sure if there are any mentioning it in the Jerusalem Talmud, nor where one might find more discussion of the idea in later Rabbinic works) that mention the idea of a soul mate, but I don't recall them off the top of my head.