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