Merge multiple "is exists" queries into one statement
UNION removes duplicate rows from the overall result, UNION ALL will preserve them. 

To know from which table the result came, add a constant value:

SELECT 'one' as source, 
       1 
FROM table_one 
WHERE id = 'a'
UNION ALL
SELECT 'two',
       1 
FROM table_two 
WHERE id = 'aa'