Yes, employers may do this. If you are performing different types of tasks for the same employer, it can pay you a different rate for the different work you are doing. This may be appropriate for servers, for example, who perform both serving duties (which may be paid at the lower, “tip credit” rate) and cleaning or sidework (which should be paid at the minimum wage if it occupies a substantial amount of the server’s time).
However, your employer cannot avoid paying overtime pay by calling the duties a different job. If you are an hourly worker and work more than 40 hours per week, you are entitled to overtime calculated based on the number of hours you worked at each rate. Check out this Fact Sheet.