Yes, you can practice the headstand (Sirsasana) in the morning and evening on the same day. However, the optimum time to practice Sirsasana is in the morning, facing east. After practicing headstand in the morning, you can move onto other asanas. In the evening, once again practice headstand and then proceed with your routine. This is the safest way to practice headstand. If you are new to this asana, start with it in the morning after having a warm-up of gentle body movements. When you are ready, increase the duration each day until you can stay in it for more than five minutes. You can also practice headstand late at night, just before going to sleep. This will help you reduce insomnia and keep you alert throughout the day. So, yes you can practice headstand daily if it does not affect your regular routine. However, if you have any problem or injury, then it is best to avoid headstand altogether. Also note that those who have neck problems or spine deformities should not attempt this asana. Its safety margin is low so be careful not to overexert yourself.