Why does it say "Shenayim Parim"?
Malbim (26): "Shenayim" teaches that they were not equal; 'Shnei' would teach that they are the same.
Why will they pick one of two bulls?
Malbim (22-23): They cannot say that Ba'al has power, just Eliyahu had a better Korban, so only he was answered. You can pick the better Korban!
Whose idea was this test?
Rashi (36, from Yerushalmi Ta'anis 2:8): Hashem commanded him. Radak (21) - Hashem told him, even though the verses do not record this 1 .
Radak (21, 36): Perhaps it was Eliyahu's idea. He was confident that Hashem will not return him empty; rather, He will sanctify His name in public. It says (verse 36) "uvi'Dvarcha Asisi" - i.e. if You answer me with fire from Shamayim, they will know that I do not act on my own, rather, according to Your word and command 2 . People should not say that the miracles were via witchcraft.
Radak: It is better to say that Hashem did not command him. Had he commanded him, why did Eliyahu need to pray (verse 42)? However, we can say that even though Hashem commanded, he needed Tefilah, just like he prayed for rain, even though Hashem promised it (verse 1). He prayed so Yisrael will know that he is a true Navi, and Hashem answers his prayer (also refer to 18:42:4:2). Without Tefilah, they could say that [the rain] was chance; they could say that fire from Shamayim was through witchcraft. (Also with Tefilah, they could say that it was via witchcraft! - PF)
Perhaps this refers to everything else that Eliyahu did. Alternatively, Hashem authorized him to make a test, but Eliyahu himself chose the test, i.e. bringing a Korban outside the Mikdash. Normally one is Chayav Kares for this! Eliyahu was sure that it is permitted for Hora'as Sha'ah, and Hashem will answer him. (PF)
