Select all results from two independent tables
Combine the two tables with UNION and ORDER BY timestamp. You can also add an addition (type) column so the source of a row can be identified:

SELECT 
    'comment' AS type, id, comment_content AS content, time_stamp
FROM
    comments

UNION ALL

SELECT 
    'activity', id, activity_content, time_stamp
FROM
    activities

ORDER BY 
    timestamp ;