Sum results from two select statements

Try this

SELECT PerceptionistID, SSNLastFour, SUM(CommissionPay) CommissionPay,        SUM(PTOPay) PTOPay, SUM(HolidayPay) HolidayPay, SUM(Overtime) Overtime, SUM(TotalPay) TotalPayFROM(    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)) tGROUP BY PerceptionistID, SSNLastFour

Make your query a subquery, and group at the 'superquery' level:

SELECT PerceptionistID, SSNLastFour, sum(CommissionPay), ...FROM (SELECT  PerceptionistID, SSNLastFour, CommissionPay,    PTOPay, HolidayPay, Overtime, TotalPayFROM [dbo].fnCalculateCommissionForWeekOf(@MondayOfCurrentWeek)UNION-- Need to get the following week's data and sum the two togetherSELECT  PerceptionistID, SSNLastFour, CommissionPay,    PTOPay, HolidayPay, Overtime, TotalPayFROM [dbo].fnCalculateCommissionForWeekOf(@MondayOfFollowingWeek)) AS XGROUP BY PerceptionistID, SSNLastFour

How about modifying [dbo].fnCalculateCommissionForWeekOf or creating a new function that takes a start and end date. This way you can use the same function, and same logic, to run against any span of dates, including the past week, 2 weeks, month, year etc.