Suma los resultados de dos sentencias seleccionadas

Prueba esto

SELECT PerceptionistID, SSNLastFour, SUM(CommissionPay) CommissionPay,
        SUM(PTOPay) PTOPay, SUM(HolidayPay) HolidayPay, SUM(Overtime) Overtime, SUM(TotalPay) TotalPay
FROM
(
    SELECT  PerceptionistID, SSNLastFour, CommissionPay,
        PTOPay, HolidayPay, Overtime, TotalPay
    FROM [dbo].fnCalculateCommissionForWeekOf(@MondayOfCurrentWeek)

    UNION ALL

    -- Need to get the following week's data and sum the two together
    SELECT  PerceptionistID, SSNLastFour, CommissionPay,
        PTOPay, HolidayPay, Overtime, TotalPay
    FROM [dbo].fnCalculateCommissionForWeekOf(@MondayOfFollowingWeek)
) t
GROUP BY PerceptionistID, SSNLastFour