Complex sentence: Over 6.5 million copies of "Half-Life 2" were sold at retail by December 3,2008, (not including Steam sales); as of February 9,2011, "Half-Life 2" had sold over 12 million copies.
Split into simpler sentences: Over 6.5 million copies of "Half-Life 2" were sold at retail by December 3,2008, (not including the number of sales via Steam). As of February 9,2011, "Half-Life 2" had sold over 12 million copies.