What did Elisha see?
Radak: He saw Eliyahu rise from the ground. He did not see after this, for the chariots and horses of fire separated between them. Malbim - this fulfilled Eliyahu's Tanai [to receive Pi Shenayim b'Rucho - verse 9].
What is the meaning of "Avi Avi Rechev Yisrael u'Farashav"?
Rashi citing Targum Yonasan: Rebbi, Rebbi, who is better for Yisrael through his Tefilah, than chariots and horsemen. Radak - we find that Talmidim of Nevi'im are called Bnei ha'Nevi'im.
Malbim: (a) All Yisrael ride on this chariot and horses of fire, for Eliyahu is equal to all of them. (b) Elisha understood that these chariots and horses are Yisrael's chariot from the Divine chariot, via which Hashem conducts and oversees Yisrael.
Why did he tear his clothes?
Radak citing Mo'ed Katan 26a: One is obligated to tear his clothes for [the death of] his Rebbi who taught to him Chachmah.
Why does it mention that he tore them into two?
Radak citing Mo'ed Katan 26a: When one tears for his Rebbi, he may never sew it back normally. The tear will always be seen.
