It's the classic 'quantity vs quality' discussion. By definition a duo costs twice as much as one-on-one. So either one girl in your price range, or two below what you can normally afford. While there are unicorns out there, generally you get what you pay for.
I'm new to this, but at the end of the day you want what you want. The single girl you want, ask her how much a duo will be (prices can be different than a simple 2x). Save up and get that. Anything else will just leave you wanting more. You'll be amazed how quickly you can save the money when you are highly motivated, so, .. um, email her and highly motivated yourself.
The only other advice I have is there is a reason a threesome is the good standard for make fantasies ....