29 C.F.R. § 4.169
If an employee during a workweek works in different capacities in the performance of the contract and two or more rates of compensation under section 2 of the Act are applicable to the classes of work which he or she performs, the employee must be paid the highest of such rates for all hours worked in the workweek unless it appears from the employer's records or other affirmative proof which of such hours were included in the periods spent in each class of work. The rule is the same where such an employee is employed for a portion of the workweek in work not subject to the Act, for which compensation at a lower rate would be proper if the employer by his records or other affirmative proof, segregated the worktime thus spent.
29 C.F.R. § 4.169