This is a true story.
Well ... sort of.
I own - and am a great fan of - the Uppity Women series by Vicki Leon. At one time, I had notes on well over a dozen of these feisty historical heroines that I wanted to turn into stories.
In ancient Mesopotamia, there is a historical case where a priestess and her sister were wed to a man in tandem ... and the wedding contract not only regulates all the chores and childbirth to the sister, it stipulates that when the priestess is happy, the sister has to be happy, too. This last clause is what really caught my interest, because of course, in a fantasy setting, you can make this literal requirement a mystical certainty ...
I tried to give the setting of Twice Given a sense of that era without hammering readers over the head with it. I did derive the names of the two female characters by juggling the names from the historical case in question - Taram, the priestess, and Iltani, the sister - but the other major player in this story (besides the husband) is entirely an original creation.