*** Please note*** This only applies to purchases made before April 23rd 2013
When you purchased from Steam during the early beta, the game was put into your games library and the friend copy of Don't Starve was put into your steam inventory. Gift purchases are not 2 separate copies of the game, it is one bundle of two games.
You cannot purchase two separate tradable copies of Don't Starve. Answers to Steam billing related questions can be found here: https://support.steampowered.com/kb_article.php?ref=8360-WEJC-2625