I'm not a SQL expert, but look at the "JOIN" statement. While I guess you can't techinically read from two tables at a time, you can use JOIN and sort of think of it as a single virtual table with all the fields you need. You can find examples of that in a lot of places in the vbulletin code.
showthread.php has (as part of a larger query):
Code:
"LEFT JOIN " . TABLE_PREFIX . "post AS post ON (post.postid = thread.firstpostid)"
http://dev.mysql.com/doc/refman/5.4/en/join.html