This glitch is very useful... you can get two feathers, two extra lives, two mushrooms, two fire flowers, two of anything that comes out of the ? blocks, including Yoshis (which is detailed in a separate glitch because of the crazy stuff it does). Use the pick up two items at once glitch to do... guess what... pick up two items at once! I find it easiest with a koopa shell and a P-switch because they're pretty common. Now, go under the ? with the item you want and throw the two items upwards to it (hold up and then push Y). Two items will come out, although it will probably look like one. However, when you get it, you will get it twice!
A Youtube video of the glitch can be seen here.