Never received code for second copy

When you purchase Don't Starve Together for yourself on Steam, one copy of the game is automatically activated on your Steam account (as it would normally). You will also find a giftable copy of Don't Starve Together in your Steam Inventory that you can send to a friend.

You can find more information about Steam Inventory and Gifting here: https://support.steampowered.com/kb_article.php?p_faqid=549 

If you have encountered a Steam purchase issue, or could not find the giftable copy in your Steam Inventory, your best bet is to contact Steam Support directly and they will be able to verify your purchase and provide support promptly. 

Link to your inventory, you should find the extra copy there.