Which Tzadikim does this discuss?
Radak: Chizkiyah and Yisrael.
Why are there two expressions of Simchah?
Radak #1: This is due to the great Simchah - "ha'Shir Yihyeh Lachem k'Leil Hiskadesh Chag v'Simchas Levav k'Holech ve'Chalil" (Yeshayah 30:29).
Rashi #2: If [Sancheriv's army fell] on Pesach night, like Chazal supported from "k'Leil Hiskadesh Chag" (Yeshayah 30:29), Yasisu refers to this redemption (salvation), together with Simchah of the Chag, which commemorates Ge'ulas Mitzrayim.
Malbim: The Simchah that they will rejoice in Hashem, because they see that Hashem is with them - the Shechinah will be a cloud and fire for Resha'im - will arouse revealed Masos.
How will their Simchah be expressed?
Rashi: They will sing to Hashem (verse 5).
