The Two Nations are taken into captivity separately

"The ten tribes are beyond the Euphrates till now, and are an immense multitude, and not to be estimated by numbers."
Flavius Josephus (c. 100 A.D.)

Israel and Judah in captivity by Assyrians and Babylonians.

The two separate nations of Israel in the north with capital at Sumeria, and Judah in the south with capital at Jerusalem, went downhill at separate rates.

Israel went into paganism first and God allowed them to be taken captive in two separate waves by the Assyrians. The Assyrians then took Gentile peoples to fill the cities of Israel. The Israelites never returned.

Judah worshipped God a little longer, but God sent the Babylonians to take them into captivity about 120 years after Israel. Some Jews returned after 70 years to rebuild Jerusalem.