BS”D
Are you sure that Moshiach ben Yosef is a must? I learned that while of course there is mention of Moshiach ben Yosef in the Gemara, there is no mitzvah to believe in his coming as a predecessor to Moshiach ben Dovid.
In any case, since it is not clear how much time there must be between Moshiach ben Yosef and the final Geula, Moshiach ben Yosef may have been among the tzaddikim who died al kiddush Hashem during the Churban.