Get a bird with the jjbird code or from a cage and have it kill any enemy in any level. After that, enter jjbird as a code for it to go away. When it leaves the screen, switch to any other weapon and shoot at anything on the screen, then enter the jjbird code again. One bird will appear from one side, then two or three seconds later, the original bird will return. You might not be able see it. However, if you look closely you can see two birds' wings flapping. This may require some practice. You will have to do this every time you want two birds.