For what are Uriyah and Zecharyah witnesses?
Rashi 1 : Uriyah prophesized in the days of Yehoyakim about the evil that will come on them - "Uriyah? va'Yinavei Al ha'Ir? k'Chol Divrei Yirmeyah";. Yehoyakim killed him. Zecharyah said (8:4) "Od Yeshvu Zekenim u'Zkenos bi'Rchovos Yerushalayim."
Radak: Yeshayah says, I made them witnesses [to sign what I wrote. Even though the verses do not say that he wrote, surely he fulfilled Hashem's command!] Even though they prophesized [much later] - Uriyah in the days of Yehoyakim, and Zecharyah in the second year of Daryavesh - they were in the days of Yeshayah.
Malbim: The purpose of the punishment of Efrayim was to save Yehudah; through this, they will see that Hashem is with us. Good and evil were mixed in the prophecy. Hashem commanded to sign on this parchment two witnesses who missions were opposites - Uriyah prepared about punishment in the days of Yehoyakim, and Zecharyah about consolation.
Radak: This is like Targum Yonasan, that Hashem makes them witnesses.
Uriyah was in Bayis Rishon, and Zecharyah was in Bayis Sheni. Why does the verse put them together?
Rashi, based on Makos 24b: Uriyah is a Siman for and Zecharyah. When you see Uriyah's prophecy 1 fulfilled, anticipate the fulfillment of Zecharyah's 2 (refer to 8:2:1:1). Similarly, Amos and Yeshayah prophesized about Sancheriv - Amos for the punishment of the 10 tribes "Galo Yigleh Yisrael" (7:11), and Yeshayah for the promise to [save] Chizkiyah, when he will reign (Perek 32).
Makos 24b: I.e. "Tziyon Sadeh Secharesh" (Michah 3:12). Tosfos (ibid.) - we do not find that Uriyah said so! However, he prophesized "k'Chol Divrei Yirmeyah" (Yirmeyah 26:20).
24b: This is why R. Akiva laughed when he saw a fox emerge from the Kodesh ha'Kodoshim.