sqlstring

I have a sp where I am creating a 2 SQL String.
When I am executing the sp if first string return '0' I exec 2 string.
In my resultset I am getting to sets from string1 and string2. What do I need to do in order to get only 1 dataset back if I execute second string?
example:
...
EXEC sp_executesql @SQLString1
...
-- if exact match is not found
if @@rowcount = 0...EXEC sp_executesql @SQLString2