Du kan bruge en join som denne:
$query = "UPDATE profiledata t1
JOIN profileprivacy t2 ON (t1.uid = t2.uid)
SET t1.aboutyou = '$aboutyou',
t1.quotes = '$quotes',
t2.aboutyouPrivacy = '$aboutyouPrivacy',
t2.quotesPrivacy = '$quotesPrivacy'
WHERE t1.uid = '$sess_uid'";