Question
Why Nãrãyana (Vishnu) had to pray Rudra (Shiva) to obtain supreme powers? (excerpt from Mahãbhãrata)
Answer
It's because Narayana is the soul of Rudra or one may also call Rudra is the soul of Narayana. Both are true. It is explained by Krishna himself in this chapter of Shanti Parva (in the Nãrãyaniya section):
He is the destroyer of Daksha's sacrifice and the tearer of Bhaga's eyes. O son of Pandu, Rudra should be known to have always Narayana for his Soul. If that deity of deities, viz., Maheswara, be worshipped, then O Partha, is the puissant Narayana also worshipped. I am the Soul, O son of Pandu, of all the worlds, of all the universe. Rudra, again, is my Soul. It is for this that I always adore him. If I do not adore the auspicious and boon-giving Ishana (Shiva) nobody would then adore my own self. The ordinances I set are followed by all the worlds. Those ordinances should always be adored, and it is, therefore, that I adore them. He who knows Rudra knows myself, and he who knows myself knows Rudra. He who follows Rudra follows me, Rudra is Narayana. Both are one; and one is displayed in two different forms. Rudra and Narayana, forming one person, pervade all displayed things and cause them to act. No one else than Rudra is competent to grant me a boon. O son of Pandu. Having settled this in my mind, I adored in days of yore the ancient and puissant Rudra, for obtaining the boon of a son. In adoring Rudra thus I adored my own self. Vishnu never bows his head unto any deity except his own self. It is for this reason that I adore Rudra.
I think the above passage told by Krishna clears all.
Note: “The question: Why Nãrãyana (Vishnu) had to pray Rudra (Shiva) to obtain supreme powers? (excerpt from Mahãbhãrata)” is licensed by Stack Exchange Inc (https://hinduism.stackexchange.com/); user contributions licensed under CC BY-SA.