According to Master Choa Kok Sui, this Union can Happen in Several Stages.
The First Stage is the realization by the person that he is the soul occupying various bodies; a spiritual being of divine light, love, and power. This realization is however still at the intellectual level. The first stage happens when one realizes that he is not his physical body, not his emotions, nor his thoughts; the emotions and the thoughts are only products of the soul.
The Second Stage, in this case, happens when one experiences himself as the soul, during meditation or any other spiritual practice, not being able to feel his physical body. The spiritual practitioner may have an out-of-body experience where the soul uses the other subtle bodies and is able to travel faster than the speed of light. He further might experience the nature of the soul as a being of light, traveling in all directions.
In The Third Stage the yogi, as the incarnated soul, may have the experience of being united with the higher soul. He may further experience oneness with other souls, including other human souls, animals, nature and etc.
The last stage is the experience of oneness with God, where the spiritual practitioner experiences oneness with God and oneness with all. Such a soul can be literally called a God-realized person.
The Spiritual Cord
The Spiritual Cord or the Antahkarana plays a very important role in the union of the incarnated soul and the higher soul. The extent of realization or union depends on the size of the spiritual connection which is in fact known as the Spiritual Cord. The thicker the Spiritual Cord, the more control the higher soul gains over the mind and the emotions. The weaknesses and unwholesome actions and behaviors can basically be attributed to less soul connection. Less soul connection happens when the higher soul loses control over the incarnated soul and its vehicles.
Yoga, a holistic spiritual practice requires a strong and pure character in order to achieve the desired results. Character building is thus an important part of Yoga and oneness cannot be achieved with the Higher soul while still being enslaved by lower thoughts and emotions.
Different branches of Yoga include Hatha Yoga, Bhakti Yoga, Raja Yoga, Jnana Yoga, Karma Yoga, Arhatic Yoga, Kundalini Yoga, and Tantra Yoga. The spiritual evolution of the soul and union with the Higher soul can be achieved through the different schools of Yoga. Arhatic Yoga, developed by Master Choa Kok Sui is a combination and amalgamation of different schools of Yoga and can be practiced by people across all walks of life and various backgrounds.
Source: The Origin of Modern Pranic Healing and Arhatic Yoga by Master Choa Kok Sui