I was reading another question and answer about a double weapon being used for two weapon fighting and it started me wondering about the wording for something else.
You can use a double weapon to fight as if fighting with two weapons, but if you do, you incur all the normal attack penalties associated with fighting with two weapons, just as if you were using a one-handed weapon and a light weapon. You can choose to wield one end of a double weapon two-handed, but it cannot be used as a double weapon when wielded in this way—only one end of the weapon can be used in any given round.
In TWF combat there is no mention of using double weapons. So the weapon property overrules the more general combat rule.
Now in the TWF feat it seems to still only consider having two weapons, and not having a double weapon.
So is it reasonable to say that a double weapon is allowed to be used with the two weapon fighting feat chain? It makes me want to build a character who uses TWF with a single weapon thats as large as possible. The cost of upgrading both ends of the weapon will hurt however.