How do you guarantee a trip to the last world with two whistles?

Solution 1:

I think you are mistaken -- using a warp whistle while in the warp island should always take you to World 8. The Super Mario Wiki confirms this:

If a second whistle is used while in the warp zone, then the player is taken directly to the World 8 Pipe.

There are also two warp whistles in World 1, as well as the third one in World 2. One is obtained in stage 1-3 by holding down on the D-Pad while standing on a white platform near the end of the stage. Another is obtained by using Raccoon Mario to float on over the top of the end of the Fortress stage in World 1. And the last is obtained by using the hammer item to break a rock in the top-right of the map of World 2. A video showing their locations is here: Super Mario Bros. 3 Warp Whistles