Can we give higher pay to employee with degree?

Q. Is it illegal to pay someone without a degree less than someone with the degree? In our case, two employees have the same title but do slightly different tasks. One has a bachelor’s in HR and the other does not. However, the one without a degree is doing all the HR duties and gets paid less than the one with a degree. The degreed employee doesn’t use her HR skills in her job at all.