To whistle with two fingers, you will have to keep practicing it. First, you have to choose your finger combination. Your options are right and left index fingers, right and left middle fingers, right and left pinky fingers, and a U-shape created with the thumb and middle finger, or thumb and index finger, of either hand. You can find more information here: http://www.natwilson.com/stuff/whistle.html
To whistle with two finger you need to remember that practice makes, um, louder. Cover your teeth with your lips, curl your tongue backwards, and insert two fingers on either side of your mouth. Now blow, not hard, but blow.