A secondary position is necessary if an employee is working out of more than one pay group. This is commonly seen when an employee works at more than one location. Visit Toast Payroll: Assign or Change Employee Jobs or Locations for more details.
In this example, let's call the current secondary position Staff Hourly B and the current primary position Staff Hourly A.
Navigate to the employee's profile > Position.
Select Edit for the current secondary position, Staff Hourly B, and either screenshot or copy down the settings on this page.
Select a Reason at the top. Position Update works well here.