It is the employer’s responsibility to make sure employees who work in more than one classification are properly paid the prevailing wages for each of the respective classifications and for the hours such work is performed. But making that determination as to what classifications and which hours were worked is sometimes trickier than it appears.
See here for a piece on “split classification” issues: https://www.jdsupra.com/legalnews/on-davis-bacon-prevailing-wage-projects-3481414