What is the significance of the children of both Israel and Judah being gathered together?

This is almost certainly a play on the word Jizreel (scattering or sowing), which featured prominently in Hosea 1:2–9 as an image of God scattering his people (i.e., sending them into exile). The respective uses of Jezreel in Hosea 1:4–5 and Hosea 1:11 are indicative of Hosea’s alternating between judgment and salvation, covenant curses and God’s ultimate promises. Even though God will certainly punish Israel for their spiritual infidelity and idolatry by removing them from the land he has given them, he also promises a future when he will gather (contrast with scatter) all of his people to himself and one another (Ephesians 2:11–22). This is the third and climactic name reversal, which is transformed from a symbol of judgment to one of God’s grace.¹ Hubbard writes, What is reversed is the way God deals with his people. What once he had to scatter he will again be ready to plant. Putting aside the name’s negative connotations, here it almost certainly indicates God sowing his people in the land.²

Tied to the covenant language in Hosea 1:10 (the number of the children of Israel shall be like the sand of the sea), the image of reunification here also points to covenant blessing. Thus God’s promise to gather his people on the day of Jezreel points to eschatological deliverance from the covenant curses and national death, or exile. A new, decisive day is at hand—and Jezreel becomes a paradigm or symbol for Israel as a whole.³