1) Does the tiefling racial feat "Armor of the Pit" stack with natural armor bonus from wildshape?
2) Do primary natural claw attacks from tiefling "maw and claw" racial trait benefit from two weapon fighting? (Does the offhand claw get a penalty?)
1.) No. Natural armor does not stack with natural armor. There are some items and effects which grant an "enhancement bonus to natural armor", which can increase your bonus, but just adding a second natural armour bonus will not help.
2.) Natural attacks run on their own system and do not need two-weapon fighting to work properly. If you're not wielding a manufactured weapon, your full attack allows you to attack with all your primary natural weapons with no penalty at all. You can also use any secondary weapons at a -5 penalty. If you do wield a manufactured weapon, all your natural attacks become secondary and one of your claws will be busy holding your weapon, but the normal rules for iterative attacks and two-weapon fighting don't come into play.