You can UNION ALL
from two SELECT
s that return columns that "line up", like this:
SELECT
postid as post_id
, post_content
, null as post_url
, post_user
... -- the rest of text_post columns included in the select
FROM text_post
UNION ALL
SELECT
post_id
, img_caption as post_content
, post_url
, post_user
... -- the rest of multimedia_post columns included in the select
FROM multimedia_post
Note how null as post_url
is selected from text_post
: this is necessary in order to line up the columns of the two select statements.