For U.S. wire gauge, the wires cross sectional area, which is what you want to match, decreases by about 50% when you go down by 3 guage sizes. If you had 17 guage wire, you could use two in hand to roughly match the cross sectional area. In your case, with 15 gauge, you will find that you can not fit the same number of turns in the stator two in hand, as it will be more bulky, and one in hand will yield a stator with significantly higher resistance, and hence perform significantly differently. My advice would be to see if you can trade the wire you have for the wire that is called for in the design you are using. Perhaps via E-bay, or Craigslist if in the U.S. Rich