http://forums.totalwar.com/showthread.php/50032-Unable-to-Purchase-Shogun-2-From-Steam-After-Buying-Fall-of-the-Samurai
You should be able to add the game to your cart, via the "Add to Cart" button, which is a little below the "You already own Total War: Shogun 2" text.
The text above the "Add to Cart" button will say "Buy this game as a gift for a friend". You can ignore this text. Once you've added Shogun 2 to your cart, and go to the cart, there will be two buttons, "Purchase for Myself" and "Purchase as a Gift". You should be able to choose the former to buy the game for yourself.