To pay employees twice a year (at 6-month intervals), create Employees as normal monthly employees. However, when calculating the payroll, skip the employees and leave them uncalculated. The system won't have an issue finalizing the roll with missing employees, however, the progress of the roll will not be 100%.

On the 6th payroll/6th month - or whenever you decide to place these intervals - make sure to calculate these employees in the month's payroll.