I think I see what you are asking. You can't sew the two together until you either make the quilt corners rounded to match up with the flying geese border (practice that unequal curve on scrap fabric first!), or square up the flying geese to match the quilt. You could add black to the inner sides of the flying geese unit to make it square. If it's not a problem, that would add a few inches to the border width.