Only LIMIT rows with specific number
Maybe you could try with:

WHERE c.tid = :id AND c.pid IN(0,1)


or 

WHERE c.tid = :id AND c.pid &lt;=1


but I am not sure what you want to get. Maybe an UNION is more appropriate.

-- only 20 with c.pid = 0
(SELECT *, c.id AS cid, u.id AS userid 
FROM comments c INNER JOIN users u ON u.id = c.uid 
WHERE c.tid = :id AND c.pid = 0 
LIMIT 20)
UNION
(SELECT *, c.id AS cid, u.id AS userid 
FROM comments c INNER JOIN users u ON u.id = c.uid 
WHERE c.tid = :id AND c.pid = 1)
ORDER BY c.posted ASC