Why did he ask "where is Hashem, Elokei Eliyahu"?
Rashi: I was promised that Pi Shenayim of his Ru'ach will rest on me!
Radak citing Targum Yonasan: Accept my prayer, Hashem the G-d of Eliyahu!
Radak: Where is Hashem, who does not show the power of the sign for me, like He did for Eliyahu via splitting the water?
Malbim: The first time, Eliyahu was able to subdue the element of water and split it, However, Elisha was with him; the merit of two is more than the merit of one. Elisha was promised Pi Shenayim of Eliyahu's Ru'ach, i.e. Eliyahu's Ru'ach will join with Elisha's. If so, [Elokus of Eliyahu] should come to me [in addition to my own, and also now the water should split] - and so happened.
Why does it say "Af Hu va'Yakeh Es ha'Mayim"?
Rashi citing 32 Midos of R. Eliezer Bno Shel R. Yosi ha'Gelili, Midah 1: Elisha's splitting the water was twice as great as Eliyahu's, for initially there was the merit of both of them, and now it was Elisha's merit alone. Radak, citing Midah 1 - this is like he said, "Pi Shenayim of you Ru'ach will be to me" (verse 9). We find that Eliyahu did eight wonders, and Elisha, 16.
Radak #1: A miracle was done also for him. He hit the water a second time after saying "where is Hashem, Elokei Eliyahu."
Radak #2: Some say that Eliyahu came, as he was [totally] spiritual, and he hit the water and it split, and Elisha passed.
Malbim: Also Eliyahu (his Ru'ach was with Elisha's), and he split the water.
