Why does it mention both Beis Yakov and Beis Yosef?
Radak: From when the Malchus in Yisrael was divided, Beis Yakov was called Beis Yosef. In the future [the kingdom] will return to be one.
Radak citing Bava Basra 123b: It mentions Yosef, for the seed of Esav will fall only to Yosef or the seed of Yosef.
Malbim: Beis Yakov will be a fire that burns nearby, and Beis Yosef will be a flame that burns from a greater distance than fire. Mashi'ach ben Yosef will do great acts then. Or, it is because the 10 tribes will still not be totally in Eretz Yisrael, and will still be far.
Why does the Navi give a metaphor of fire and straw?
Radak: Just like fire consumes straw, so [Yisrael] will consume [Edom] and they will be eradicated.
Malbim: Beis Esav will be like straw, then when it 'smells' (is near) fire, one piece of straw 'eats' (ignites) its colleague [the piece next to it]. So [Bnei Edom] will perish one after the other in the war of Gog, like it says in Yechezkel Perek 38, until no remnant will remain.
Where did Hashem say so?
Rashi: "Va'Yered mi'Yakov v'He'evid Sarid me'Ir" (Bamidbar 24:19).
When will this occur?
Malbim: It will be at the time of the war of Gog and Magog.
