I completely, completely agree with Nyc. What if it all starts out good but then the new woman realizes she's just not into him? Does that mean you and she would have to break up too? A women you can share sounds like a great idea, and some people do indeed find that, but to have it as a requirement is unfair and unrealistic if you want something more than totally casual sex with someone you can discard if it doesn't work out exactly the way you want.
Stand up for yourself and tel him that what you'd like to do is look for a real woman, not a mythical creature.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.