He ALSO has Combat Expertise. And you can use both at the same time.
Wooo! Attack penalties galore!
This assumes hitting the opponent is the objective.
Let's take a close look at this one.
Usually, one of the objectives is to provide flanking (for the rogue).
Which would work better if he could hurt people along with the rogue, I would imagine.
Another is staying near the combat to provide another character extra hp through 'shield other'.
Actually contributing to the combat with damage seems to me more useful than being an HP battery.
A third is delaying the opponent until one of the 'strikers' can get to them.
How? Does the player have some class feature that prevents people from going past him?
And why assume you can't hit the opponent in the first place?
Alright, let's see.
For this example, let's look at a hypothetical level 6 fighter
Now, Int of at least 13, which leaves less points for Str, Dex, and Con.
You have to have decent Con in order to be an HP battery. You have to have good Dex for the two weapon stuff. Guess you take a hit to strength.
Which... just happens to be the stat contributing to your AB.
So let's say you have a Str of 16. That leaves you with enough points for a starting Dex of 16 (can be bumped to 17 at level 4 to qualify for ITWD), Con of 14, Int of 14, and Wis 8 Cha 8. This, of course, assume a 32 point buy, which may not be the case in your game.
You have 7 feat so far.
Combat Expertise, Two Weapon Fighting, Two Weapon Defense, and Improved Two Weapon Defense are already given. That leaves 3 floating.
Assuming you have a +1 or masterwork weapon, and the Weapon Focus feat, you have an AB of 6+3+1=11.
A -4 penalty from Fighting Defensively reduces that to +7. The -2 penalty from two weapon fighting reduces that to +5. A -11 penalty (Combat Expertise) improves that to +0.
Assuming your enemy has an AC of, say, 18 (No Dex, full plate), then you hit 35% of the time when fighting defensively with the double weapon. Flanking with a rogue improves this to hitting 45% of the time. Assuming you have a +1 weapon and the Weapon Specialization feat, you deal 1d6+3+1+2= 9.5 damage.
To be fair, you get two attacks. I wouldn't trust my own calculations, but it seems that you only hit both times 12.25% of the time. Not that great.
On the bright side, your AC is about 18 normally and 22 when fighting defensively. At level 6, the enemy can have an AB of about 11 which means he'll hit you half the time too. Except he'll probably be doing more damage...