If the only difference between reviews and articles is a matter or categorization, and you sometimes want to mingle and sometimes separate the categories, then you made a mistake when you created them as separate tables.
Much simpler to have a single table where type 0 means article and type 1 means review:
SELECT FROM table; // gets all records
SELECT FROM table WHERE type=1; // gets just the reviews
So you wouldn't have to ask this question.
With mysql 4 you can perform a union of tables:
(SELECT FROM articles)
UNION (SELECT FROM reviews)
ORDER BY hits DESC LIMIT 10;
... and get what you want. There are probably other ways that a SQL guru might suggest, including temp tables. But fixing your data model up front is probably a better way to go about it.
http://www.google.com/search?&q=normalize%20sql%20data%20model
