A handstand split is an extremely difficult pose to execute in gymnastics and yoga. It is centered around flexibility, balance, strength and control. You must be able to master all four of these things in order to successfully perform this move.
Make sure you can do a regular split. If you can't, then you will not be able to do one during a handstand.
Practice doing a regular handstand. If you are trying to execute this advanced maneuver you should have already mastered the basic handstand. You should know how to control the center of your body (your abdomen), which keeps you from falling. Once you are comfortable and can hold a handstand for a period of time, you can move on to the next step.
Use a crash mat, to help give you cushion in case you fall a few times. Do a regular handstand and hold it. After you have your balance, and you feel secure in your pose, begin pulling your legs apart. One leg should go towards the front of your head, and the other in the opposite direction. You should continue until you are in a full split.
If you need assistance maintaining the handstand split, move closer to a wall. Allow enough distance between you and the wall that your leg will be able to extend fully. While in the handstand position, widen your legs until you are in the split again; this time the front leg should touch the wall. The wall should serve as a means of support. Keeping one foot on the wall while in the split will allow you to get comfortable in the position. Soon, you will adjust and be able to do it without the extra support.