the two divisions that would go off duty on the Sabbath are to guard the house of the LORD for the king.
Six years Athaliah tyrannised. We have not a particular account of her reign; no doubt it was of a piece with the beginning. While Jehu was extirpating the worship of Baal in Israel, she was establishing it in Judah, as appears, Ch2 24:7.
Commenting on 2 Kings 11:4-12
And two parts of all you that go forth on the sabbath,.... The course that went out of service at the end of the sabbath; these were not suffered, not any of them, to go into the country, but were detained for the present purpose, and divided into two parts: even they shall keep the watch of the house of the Lord about the king...
And two parts of all you that go forth on the sabbath, even they shall keep the watch of the house of the LORD about the king. (h) Whose charge is ended.