Coming Up to Handstand with Both Legs
Coming up to Adho Mukha Vrsksasana (Handstand) with legs together takes time, patience, and practice. To begin, there are several ways to feel the action of coming up with legs together. The first is to practice moving from Halasana (Plow Pose) into Salamba Sarvangasana (Shoulderstand) with both legs together. You can also practice slowly coming in and out of Salamba Sirsasasana (Headstand) with legs together. By practicing these, you will feel the abdominal strength that it takes to complete the action and you'll get a sense of your own balance.
Try practicing Handstand at a wall and coming halfway down (to a right angle) with your legs straight and together. Then go back up. Experiment with how far down you can come without losing control. Pay attention to your breathing, keeping it smooth and relaxed the whole time. If this is too difficult, start with a very small movement and gradually increase the movement over time. When you feel unsteady, engage the abdominal muscles (especially as they move toward the spine on the exhalation) to support and stabilize the front of your spine. Keep these strengthening poses as part of your practice toolkit.
You are correct that the action of coming up to Handstand with both legs together involves shifting the balance of the pelvis over the arms and head. The simplest pose that teaches this movement is Marjarasana (Cat-Cow Pose). Come to hands and knees. Bring the hands to shoulder width and place slightly forward of the shoulders. The knees are directly under the hips, hip width apart. Marjarasana is usually practiced with an exhalation as you round your back and an inhalation as you make it concave. In this case, however, I think it is better to exhale for both movements and inhale in between. Exhale as you shift the sitting bones and tailbone up toward the ceiling, roll the shoulders back and lift the chest and head. Exhale again as you come to the cat position. Bring the tailbone toward the pubis and round the back and shoulders. Tuck the chin toward the chest.
Then practice the same movement in Downward Facing Dog (Adho Mukha Svanasana). Come up onto the balls of your feet and take your sitting bones as high in the air as you can and then come down onto your heels. Try to resist the sitting bones dropping as your heels descend. Do this by initiating the action from your pelvis rather than your feet.
Gradually walk your feet closer to your hands, continuing to practice this pelvic movement. Eventually, you will be in Uttanasana (Standing Forward Bend). As you bring your sitting bones up, feel the weight come onto your arms and use a strong smooth action of your abdominal muscles with your exhalation to lift you into Handstand. At first you will probably need to hop to come up. Find the smallest hop that you need.
Like kicking up with one leg, at some point you will find that you "get it." Stay connected to your breathing and gradually the action will become smoother. Eventually it will develop the effortless quality you would like and you will feel like you are flying.
The late Esther Myers' 10 years as a student of Vanda Scaravelli inspired her to find her own unique, organic approach to yoga. Esther taught classes across Canada, Europe, and the United States before her death from cancer in 2004. She left behind a practice manual for beginners and a book titled Yoga and You, as well as two videos, Vanda Scaravelli on Yoga and Gentle Yoga for Breast Cancer Survivors.