|Darksol the Painbringer|
Darksol, so you're saying that because the feat specifically references two-handed weapons, and because when you are dual-wielding you are treated as having a one-handed weapon and a light weapon, not a two-handed weapon, that you cannot dual-wield a double weapon in conjunction with this feat?
If that's what you're saying, than I don't personally believe in that interpretation for a moment.
Don't believe it if you want. I don't believe the factor that you cannot attack with the Shield while using a two-handed weapon, but according to Owen, you can't do that with this feat, because attacking with the Shield makes it something that isn't a Shield (a Martial Bludgeoning Weapon).
The same concept applies here. You can't attack with a Two-handed Weapon with the Double property as a Double weapon, because attacking with the Two-handed Weapon as a Double weapon makes it something that isn't a Two-handed Weapon (a One-handed and Light Weapon).
If it makes you feel any better, I don't agree with it either. I'm just appropriating what Owen's interpretation is in relation to the question posed. That being said, you still can't TWF with a Two-Handed Weapon and a Shield because of the unwritten rule, and as you've already demonstrated, trying to TWF with a Double Weapon while using a Shield is quite questionable. I find it better to err on the side of "It's not happening." Even if it is plausible (though by Owen's argument, it shouldn't be), I have no idea why you would do that, considering how extremely feat intensive you become for doing so.
Let's take a typical Human Fighter, shall we?
1. Power Attack, Shield Focus, Shield Brace
2. Two-Weapon Fighting
3. Weapon Focus
4. Weapon Specialization
5. Shield Specialization
6. Improved Two-Weapon Fighting
7. Double Slice
8. Greater Weapon Focus
9. Greater Shield Focus
10. Two-Weapon Rend
11. Greater Two-Weapon Fighting
12. Greater Weapon Specialization
13. Greater Shield Specialization
14. Penetrating Strike
15. Hammer the Gap
16. Greater Penetrating Strike
That is an extreme amount of feats that you take to optimize being able to wield both a Double Weapon and a Shield at the same time. The good news is, it comes online by 2nd level, and only gets better. The bad news is, you don't really spend your feats on anything else except that until 17th level.