User forum posts, read from two different tables

[crazy/man]

I have two forums, using same database but under different prefix.

 

forum1 works under prefix f1_

forum2 works under prefix f2_

 

how to combine users posts from both forums, so when user posts on forum1 , he will have increased forum posts count on second forum too.. if you can understand me.

 

i tried something like this.. but it doesnt work.

 

	$posts1=mysql_fetch_array(safe_query("SELECT postID FROM ".f1_."forum_posts WHERE postID='".$id."' AND poster='".$userID."'"));
$posts2=mysql_fetch_array(safe_query("SELECT postID FROM ".f2_."forum_posts WHERE postID='".$id."' AND poster='".$userID."'"));
$posts=$posts1+$posts2;