Your query is correct. You might have used wrong column name, please check.
If still problem persist post your table structure too. I have created a sample query use this for your reference
DECLARE @Table1 TABLE (EmpNumber VARCHAR(20), value INT, WeekPeriod INT, YearPeriod INT)
INSERT INTO @Table1 VALUES
('101',1,1,2012),('101',1,2,2012),('101',0,3,2012),
('102',1,1,2012),('102',1,2,2012),('102',0,3,2012),
('103',1,1,2012),('103',1,2,2012),('103',0,3,2012)
DECLARE @Table2 TABLE (EmpNumber VARCHAR(20), value INT, WeekPeriod INT, YearPeriod INT)
INSERT INTO @Table1 VALUES
('101',1,1,2012),('101',1,2,2012),('101',0,3,2012),
('201',1,1,2012),('201',1,2,2012),('201',0,3,2012),
('202',1,1,2012),('202',1,2,2012),('202',0,3,2012),
('203',1,1,2012),('203',1,2,2012),('203',0,3,2012)
SELECT
SUM(ISNULL(value,0)) AS Val
FROM
(
SELECT
*
FROM
@Table1 T1
WHERE
T1.EmpNumber ='101'
UNION
SELECT
*
FROM
@Table2 T2
WHERE
T2.EmpNumber = '101'
) Val
Regards
Praveen