First whistle is 1-3!
In World 1 Stage 3, find the white platform in that stage, stand on it, and duck for about 5 seconds or so. Mario will fall through and be behind everything in the stage. Run as fast as you can to the end of the stage.
the second one is in the 1st fortress!
In the fortress of World 1, fly up to the ceiling and make your way to the right. Once you move all the way
to the right, press up. You'll find yourself in a secret room with a treasure. The treasure contains the
second secret whistle.