You can apply two salary rates to an employee's single pay period by adding an additional pay type.
Here's how:
In your left navigation bar, go to Payroll, then Employees.
Find and click the employee's name.
Hit Edit employee.
Under How much do you pay this employee?, choose Add additional pay types if you haven't selected any other pay types, or use the Pencil (✏️) icon if you have.
Press Add an hourly rate.
Enter a description and specify a dollar amount for their hourly rate.
Select Done.
Now when you create their paychecks, the selected rates will appear in your table. All you'll have to do is enter the appropriate hours.