According to the author of 2 Chronicles, there were not enough priests to finish all the work of preparing the offerings, so their helpers, the Levites, who were better prepared, helped them until they were finished. The Levites were ready and eager to do the right thing and help out.
2 Chronicles 29:34: But the priests were too few, so that they could not flay all the burnt offerings: wherefore their brethren the Levites did help them, till the work was ended, and until the [other] priests had sanctified themselves: for the Levites [were] more upright in heart to sanctify themselves than the priests. [did help...: Heb. strengthened them]
It states that the priests were numerous and sufficient for the task of preparing the offerings, unlike in 2 Chronicles 29:34 where the priests were too few.
2 Chronicles 35:11: And they killed the passover, and the priests sprinkled [the blood] from their hands, and the Levites flayed [them].