Sri Varaha Avataram – Varaham can swim in water and cannot submerge in the waters. Even if we accept that Varaha can submerge in waters in the Sri Varaha incarnation, why do we offer Korai Kizhangu (a type of root) as naivedyam to Sir Varaha Perumal? When brothers Ravanan and Kumbhakarna, Sishupala and Dantavaktra were killed in one/same respective incarnations (as Rama and Krishna), why did Perumal take two incarnations to kill the brothers Hiranyaksha and Hiranyakashipu? Even when they are not brothers, what is the reason for them to have ‘Hinranya’ in their names?

Sri Varaha is not the ordinary Varaha. He is Perumal, Mahavarahan. Swami Desikan says ‘kunA pOtrI’. His capabilities are uniquely different from that of the ordinary Varaha.
Swami Desikan says in his Bhu Stuti – “vyōmātilaṅghini vibhōḥ praḻayāmburāśau vēśantalēśa iva mātumaśakyamūrtēḥ”. That is, the dimension or size of Mahavaraha is such that, the deluge waters (Pralaya jalam) were like a pond in comparison. As a result, not sure if there could have been any reason for Hime to submerge! Since His form was colossal, He effortlessly rescued Bhumi Devi.
Hiranyaksha and Hiranyakashipu were the sone of Diti. They were brothers. Perumal took two incarnations to grace the world with two different experiences. In Varaha’s incarnation, through Piratti He preached Varaha Purana, several means, and Varaha Charama shloka. In the other incarnation, to demonstrate that He rushes to the wailings of His devotees and to show them His affection for them. It appears that through these two incarnations, Emperuman revealed two different chief auspicious traits of His.