UNION ALL LEFT JOIN
SELECT
comments.*, users.company, users.contact_person, users.email
FROM
comments
LEFT JOIN users ON users.user_id = comments.user_id
WHERE
comment_id = %s
UNION ALL
SELECT activity.*, users.company, users.contact_person, users.email
FROM
activity
LEFT JOIN users ON users.user_id = activity.user_id
WHERE
comment_id = %s
ORDER BY
timestamp ASC