One of the most important questions we can ask has to do with the gemara in Succah 52A, which is (I believe) the only place in the Shas which mentions Moshiach ben Yosef. There, the gemara tells us that Moshiach ben Yosef will die in the war of Gog Umagog (according to one opinion). Interestingly (and we'll see more about this later), the gemara tells us that when Moshiach ben Dovid sees that MBY has died, he asks Hashem that he be spared, and not suffer the same fate. To this, Hashem responds that Dovid Hamelech (MBD's forebear) has already davened on his behalf and Hashem has already agreed to grant MBD life.
1. Why does Moshiach ben Yosef have to die (according to this opinion)?
2. Why is Moshiach ben Dovid concerned that he might die, as well?
There is a commentary on the gemara called "Einei Shmuel" who explains both of these issues.
here is my free translation:
"The Maharsha writes that when he [MBD] sees that Moshiach ben Yosef is killed in the war of Gog Umagog, he will be fearful lest he be killed as well. see there. We need to understand the concept of this fear - just because this one [MBY] is killed, does he [MBD] also have reason to fear this?
However, this is explained in Asarah Mamaros (ten teachings), the teaching of 'chakor din' part 4, chapter 16, that the concept of Moshiach ben Yosef being killed is because of the sin of his ancestor - Yeravam ben Nevat. He [MBY] will be killed to atone for him. see there. This is also explained in Megaleh Amukos 158, 159 and 252. see there.
And examine Emek Hamelech page 25, that the soul of Yeravam flew out of Yeravam right before the sin, and that soul will be in Moshiach ben Yosef in the future.
Behold they [the sages] said in Shabbos (56B): R' Y. said in the name of Rav: When Dovid said to Mefiboshes [the son of Shaul] that he and Tziva should split the field, a heavenly voice came out and said 'Rechavam [Dovid's grandson] and Yeravam will split the kingdom [of the Jewish people].' R' Y. said in the name of Rav: If Dovid had not listened to the Lashon Hara (gossip), the kingdom of Dovid would not have been split, and the people of Israel would not have served idols, and we would not have been exiled from our land. see there in Rashi.
Based on this, the main cause of the sin of Yeravam was Dovid. Being that Moshiach ben Yosef is killed for the sin of Yeravam; which ultimately was caused by Dovid, we can understand Moshiach ben Dovid's fear that he would also die. To this Hashem reassures him that he will live."
end piece of "Einei Shmuel."
We see many remarkable things here. I would like to point out that MBY is not just an individual, but also a process. The same is true of MBD. So when we see that there is an idea of death surrounding the individual MBY, we can also say that the MBY process involves death. When we speak of the process of MBD, in contrast, we can see that there could be a concern of death in that process, as well (just like MBD himself is concerned), but we are assured that there is no death there.
Another example of this is in the redemption of Mitzrayim (Egypt) - the last two plagues - darkness and death of the firstborn - these two correspond to the last two sefiros - Yesod and Malchus. Paralleling this, MBY corresponds to Yesod and MBD corresponds to Malchus. It is remarkable to note that during the plague of darkness 4/5ths of the Jewish people died. This corresponds to Moshiach ben Yosef and his process. When it came to the death of the firstborn - the Jewish firstborns should have also died! They were only saved because Hashem gave them a specific directive to place the blood on the doorposts - only this act would spare them of death. This corresponds to the aspect of Moshiach ben Dovid - where death could happen... but doesn't.
I will leave you now with a few more questions we need to ask:
According to the opinion (the GR"A) who says that MBY will not die, what happens when MBD comes - does MBY step down? do they both rule? Can there be two kings with one crown?
What is the depth of what we're witnessing today - with the ingathering of the exiles it seems that the Ashkenazic and Sefardic streams are mixing in Israel to some extent - do the rules still hold true? What happens when MBY-Ashkenaz and MBD-Sefard come together?
Can we surmise which stream will produce the actual Moshiach ben Yosef and the actual Moshiach ben Dovid?
What is the significance of the fact that Yeravam (a fallen MBY) was the leader of the ten tribes, as opposed to Rechavam (MBD) who was the leader of Yehuda and Binyomin? Can we see a parallel that can apply to MBY-Ashkenaz and MBD-Sefard?
Why is it that in the geulah of Purim - we seem to have bypassed the problem of the Jewish people's death - which Haman attempted - and which would correspond to MBY - and we went straight to the aspect of MBD where the Jewish people are ascendent and saved from death?
What does that mean for us, as we face off with the same Persian foe? Can we expect things to be similar today?