It is great that Tenshug(long life puja) is being done for these two great Lamas and Tulkus of very great Masters - Kabje Yongyal Rinpoche and Domo Geshe Rinpoche.
Yes, as Mana has clearly said, Tulkus are to be highly treasured because we depend on them to continue to turn the wheel of Dharma and benefit countless beings. If Tulkus pass away, we would have to await a period of time before a new reincarnation is born and recognized and is trained and grows into adulthood and is ready to turn the wheel of Dharma again.
In fact, 7 year old Domo Geshe Choktrul Rinpoche is the third incarnation of Domo Geshe Rinpoche; the second Domo Geshe Rinpoche had passed away at a relatively young age, but not before he had done a tremendous amount of work to ensure the continuity of Dharma. Escaping from Tibet after suffering severe torture in the hands of his Chinese captors, he was unfazed and continued his work ,in India, to preserve treasures of Tibetan Buddhism. The first Domo Geshe Rinpoche was a highly attained Master of legendary fame. He was a great devotee of Dorje Shugden.It was in his Dungkar Monastery that Dorje Shugden, Kache Marpo and Namkar Barzin manifested through the famous oracle there. Thus much is expected of the third Domo Geshe to fill the shoes of his illustrious predecessors.
These two incarnate lamas are thus highly precious Holy Beings to ensure the continuity of the pure lineage of Je Tsongkapa's teachings.
Thus it is important to perform the Tenshug in its completeness, to generate powerful energy, merits and blessings that will create the cause for these two Lamas to live long and continue to turn the wheel of Dharma.