1)
When were they one Beis Av?
1.
Vilna Gaon: This was in the days of Ezra. In the days of David, each was a separate Beis Av 1 .
2.
Malbim: It was in the days of David, like this Perek discusses.
1
Malbim: This is wrong. This Perek discusses the days of David! (Perhaps it says "Yihyu", to teach that later they became one Beis Av! - PF)
