AGAINST() only accepts a string literal:
AGAINST takes a string to search for, and an optional modifier that indicates what type of search to perform. The search string must be a string value that is constant during query evaluation. This rules out, for example, a table column because that can differ for each row.
-- http://dev.mysql.com/doc/refman/5.6/en/fulltext-search.html
This rules out any construct I can think of where an ordinary query could be used to join tables based on a fulltext index match.
...however...
You can wrap the fulltext query in a stored function:
DELIMITER $$
DROP FUNCTION IF EXISTS `best_fulltext_match` $$
CREATE FUNCTION `best_fulltext_match` (search_string TEXT) RETURNS INT
DETERMINISTIC
READS SQL DATA
BEGIN
RETURN (SELECT id FROM title_table WHERE MATCH(title) AGAINST(search_string)
ORDER BY MATCH(title) AGAINST (search_string) DESC LIMIT 1);
END $$
DELIMITER ;
Now...
SELECT best_fulltext_match('your title here');
...returns the id of the best match according to the fulltext index in title_table, and this function will accept a variable, no problem.
You could then use this to update your random_title_table.
UPDATE random_title
SET title_id = best_fulltext_match(title);
The function is invoked once per row in random_title, and gives the fulltext search a static value to search with.
You could also use that same function when creating new entries in random_title.
