dynamic Sql statement using Union operator(very urgent!!!)

I m facing problem while using union operator with dynamic sql statement...
here is the error "Incorrect Syntax near UNION"
Here is my code example....
Set @FirstSql='select * from tableA'//complex query here
set @SecondSql='select * from tableB'//Complex query here
Exec(@FirstSql + @SecondSql)
union
Set @ThirdSql='select * from tableA'//complex query here
set @FourthSql='select * from tableB'//Complex query here
Exec(@ThirdSql+ @FourthSql)

Say you have a script:
SELECT * FROM Table1
SELECT * FROM Table2

This script will return two resultsets.
Add the keyword UNION before the second SELECT. The script will now return one resultset (provided that the columns match up):
SELECT * FROM Table1
UNION SELECT * FROM Table2
In other words, UNION is not a control-of-flow operator!

Hi,
Do you mean something like this:declare
@q1 varchar(900),@q2 varchar(900),@q3 varchar(900),@q4 varchar(900)set
@q1='select 1'set
@q2='select 2'set
@q3='select 3'set
@q4='select 4'exec
(@q1+@q2+@q3+@q4)exec
(@q1+' UNION ALL '+@q2+' UNION ALL '+@q3+' UNION ALL '+@q4)