Mix light brown sugar, granulated sugar, heavy cream, butter, water, and salt in a heavy bottomed saucepan. Cook over low heat, stirring constantly with a wooden spoon, until sugar dissolves. Stir in pecans and cook over medium heat until mixture reaches the soft ball stage, 238-240 degrees F on a candy thermometer. If you spoon a drop of boiling syrup into a cup of ice water, it will form a soft ball that flattens easily between your fingers.