There's a whistle in World 1 and a whistle in World 2. When you use one whistle, it takes you to that little warp island with three connected sections of pipes that warp you to different worlds. The top row has worlds 2-4; the middle row has worlds 5-7; the bottom row has world 8 - the last world.
Like I said before, when you use one whistle it takes you to the top row. Then if you use a second whistle while you're still on the warp island, it sometimes takes you to the middle row and sometimes takes you to the bottom row. This has always frustrated me like crazy!!
Is there a way to always guarantee you'll make it to the bottom row (with the warp to the last level) by only using two whistles?