2-Weapon Fighting vs. 1-Weapon Fighting

[Conaill]

If fighting a +24 AC the character tends to swap to a single weapon and usually hits once inflicting an average 19 points of damage.

So put him up against a +24 AC fighter with a single weapon, and all his specialization goes out the window!

Just make a 1/2 orc opponent with the same exact same stats (maybe put the 15 in Int?), using a longsword and shield. Now equip him with the same amount of gear. Your guy has a +2 flaming and a +1 flaming bastard sword, right? That's 26670 gp right there. +3 full plate and a +3 large shield adds up to +16 AC with 6850 gp to spare. Or a +3 full plate, +2 large shield and +1 ring of Protection: 9850 gp to spare. That allows you to add a nice +2 sword as well. Or you can get another +1 to AC (sword of Defending, +2 ring of Protection, Amulet of Natural Armor...), or some other nice item.

And then of course, you get to pick three extra feats that your guy had to spend on Ambidexterity, 2WF, and Exotic Prof: Bastard. Dodge and Expertise if you further want to trick out his AC. Improved Disarm to get rid of his nice flaming toys (remember: a bastard sword is a medium sized weapon!).

Nasty tricks: armor of Fire Resistance, or a mage casting the spell would sure annoy the flaming sword guy. Glamered armor could make him use the less effective 2-weapon approach because he doesn't realize he's fighting against a 24+ AC opponent.

 

[Gromm]

Ummm... so hes got a +2 flaming broadsword and a +1 flaming broadsword then?
Did someone in the party make them or did you give them to him?

The gp value on the swords alone is 26000 gp, the wealth limit for a 6th level character is 17000gp. I'm assuming he has other magical gear.
This might be your problem more than 2 swords. Hes way too well equipped for his level. He should be a min of 8th level (27k gp limit) before he could afford the swords alone.

Against low AC foes he's going to dominate, against anything with ACs in the mid 20's (like other fighters) hes going to start having problems. Throw about 4 lvl 3 fighters with Phalanx Fighting (from LoD) against them. As long as they stay within 5' of each other they should have an AC of 23 (+8arm, +2shield, +1PF, +2cover, +1dex), with some good tactics he shouldn't be too able to get through.

Or just make a fairly good NPC fighter and have him drink a potion of haste before the fight. Without any magical gear his AC will be at least 25, more with any sort of magic armor or gear.