What is the exact Query To add two rows from different coloums?
Several options such as:

SELECT (SELECT SUM(follow) FROM fb WHERE name = 20)
    + (SELECT SUM(follow) FROM tw WHERE name = 20);


Or: 

SELECT SUM(follow) FROM (
    SELECT follow FROM fb WHERE name = 20
    UNION ALL 
    SELECT follow FROM tw WHERE name = 20
) as u;


The query in the question has one problem:

SELECT tw.name,fb.name,SUM(fb.follow+tw.follow) 
FROM fb, tw 
WHERE fb.name = 20 AND tw.name=20;



values are counted twice because fb CROSS JOIN tw (i.e. fb, tw) duplicates each follow value


This is the result of the CROSS JOIN query without SUMafter duplication:

id  name    follow  id  name    follow
1   20      1       1   20      3
2   20      2       1   20      3
1   20      1       2   20      4
2   20      2       2   20      4


Note that values (all 1) have been replaced by 1, 2, 3, 4 because it is easier to see the problem.