Albert Barnes Commentary 2 Chronicles 35:15

"And the singers the sons of Asaph were in their place, according to the commandment of David, and Asaph, and Heman, and Jeduthun the king`s seer; and the porters were at every gate: they needed not to depart from their service; for their brethren the Levites prepared for them." — 2 Chronicles 35:15 (ASV)

They were not to depart - The singers and porters remained at their posts, while other Levites sacrificed for them and brought them their share of the lambs.