Swordmage does not always make a great main defender, but it doesn't do bad a secondary. They're great at taking out ranged attackers, which is something other defenders like fighter have a rough time with. Assault gets to teleport right in there and smack them which when followed by booming blade, locks an artillery down pretty good. Shielding only mitigates damage, but it has a decent selection of powers that help them add more to thier aegis. This was the first class I thought of when you mentioned warforged.
Anyway, on the mention of fighter, adding one to a party never hurt it. They do a great job using striker as a secondary role when you build them right, which means they still have a lot to contribute when two defenders wind up being a crowd.
Either of these are suggestions I have. They're based on bias I have from what I've played, but I imagine the combo couldn't hurt your group.
A Fighter and a Berserker would be pretty good. Fighters can deal solid to very good damage, and the Berserker can flip to striker mode once things are clearing up, to help end encounters quicker.
Wardens make very good defenders, and are built to basically take a small area of the battlefield and lock it down. Fighters and aura defenders can do the same, but Fighters have a harder time locking down multiple opponents and doing very good damage.
A Warforged warden is pretty flavorful, imo, depending on how you're doing warforged, and a warden and a berserker are both primal, so that gives a bit of similarity between the two (same with fighter and berserker on the martial side).
Swordmages, as was noted, can make very good second defenders, and a Shielding swordmage can help mitigate a fair amount of damage, though they don't deal much damage on their own. Assault swordmages can do alright on the damage side, but their defender mechanic isn't very good.
Paladins are, of course, very good all around defenders, and can also do a lot of good damage.
You've got a lot of options, but really any two defenders is going to be good. The Fighter works well as both a primary and secondary defender. Regular weaponmasters can always just choose not to mark the enemy, while Knights tend to work more like Wardens, locking down one small area of the battlefield.
"Not only are you wrong, but I even created an Excel spreadsheet to show you how wrong you are." --James Wyatt, May 2006
the reasono why is solo enemies, or encounters that whittle down to one elite.
marks overwrite one another, so in that kind of situation one oof the two defenders is going to be left out.
defender auras only partially cancel each other. if your aura is overlapping another one, the enemy in both auras can trigger both punishments if it shifts or attacks a non-defender. the downide is that the enemy has its choice of defender to attack. not much of a downside.
Defender Aura reads ..
Effect: You activate an aura 1 that lasts until you end it as a minor action or until you fall unconscious. While in the aura, any enemy takes a -2 penalty to attack rolls when it makes an attack that does not include among its targets either you or an ally of yours who has this aura active. Marked enemies are not subject to this aura.
so if it attacks the other defender it isn't -2 to hit.
Battle Gaurdian reads ..
Trigger: An enemy subject to your defender aura either shifts or makes an attack that targets an ally of yours but not you or an ally who has an active defender aura.
so it doesn't trigger if the attack is the other defender. ok.
but if the attack is against any of the non-defenders, you both smack it. if the enemy shifts, you both smack it.
now you aren't fighting each other to apply marks, you're working together to provide extra discincentive to shift or attack the weaker party members.
you've got a Bard, right? Knights typically have maximized MBAs and access to power strike. Have your Bard push with staggering note, you take a swing and then the enemy is pushed out of your aura and .. right into your buddy's aura.
put your knights adjacent to each other and you get a 5x3 wall of aura. space them a little for 6x3 doing a pincer movement around that is tricky and Defend the Line makes taking an opportunity attack insufficient to break through.
one last thing, you might even want to do something like have one Warforged Knight and one Eladrin Knight with Melee Training [INT] then at second level retrain that to MC Swordmage, take Intelligent Blademaster and then eventually Eladrin Swordmage Advance .. this variety prevents the characters from playing too similarly and adds some character to the relationship [think Abbot and Costello, R2D2 and C3P0 - fun right?]
normally Eladrin Knights are kinda meh, but with another fighter they become more interesting. if you are in a corridor type fight, teleport over the front line and into the enemy artillery and controllers while the warforged holds the line. or if something in both your auras attacks someone else or shifts, use Feywild Guardian to teleport into a flank before you both make your punishment attacks.
could be an interesting and effective combination, even if it isn't amazingly optimal.
Battleminds make fun second defenders, particularly after lightning rush takes over for mind spike (level 7). If an enemy (even one not marked by you...) obeys your co-defender's mark/aura, you can lightning rush over, smack it, and redirect its attack against you...triggering your co-defender's punishment as well. If you pick up Forceful Reversal as well, or hybrid ardent for unsteadying rebuke (and a MBA!), you can attack twice a round even if your target is obeying your mark and attacking you.
Battleminds are notably bad at stickiness and at locking down multiple targets, but their punishment can be brutal until they run out of power points!
I'd go with Battlemind|Swordmage(example in my in-play thread in my sig) and Fighter who can slide on an MBA, either of which can make good Warforged.
Fighter marks an opponent, the opponent attacks the Fighter, you interrupt with Lightning Rush - on a hit, the opponent attacks you, the Fighter interrupts and attacks the opponent and slides him away from you on a hit so he can't attack you.
Long waits for tricks are only bad for one trick builds. The parameters mwao set are simple enough that almost the entire build is left to define. Fighters are amazing (even for multi-enemy control despite what an above poster claimed) and paired up with another defender a battlemind can really shine.