If an incorporeal creature is attacking another incorporeal creatures that is inside a corporeal wall, does the wall give cover or just concealment? What if the wall can be seen through?
Technically the incorporeal special quality says that such a creature inside an object has total cover. It doesn't say only against creatures outside the object.
That said, it'd be very reasonable to ignore that for some hot wraith on wraith action. Objects don't slow incorporeal creatures down or anything, as long as it's thin enough. Personally I'd rule there's no obstruction for them.
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Nothing grants the attacking incorporeal creature the ability to see through the wall, and nothing about the attacker being incorporeal ignores the other rules associated with being incorporeal.
It can sense the presence of creatures or objects within a square adjacent to its current location, but enemies have total concealment (50% miss chance) from an incorporeal creature that is inside an object. In order to see beyond the object it is in and attack normally, the incorporeal creature must emerge. An incorporeal creature inside an object has total cover, but when it attacks a creature outside the object it only has cover, so a creature outside with a readied action could strike at it as it attacks.
The creature within the wall has total cover and full concealment, whether the attacker is incorporeal or corporeal.
Actually in our case, we do have an ability to see through stone. So, the question is still valid.