What exactly are the implications of this feat?
- Do the item enhancement bonuses from two implements stack?
- Do the properties/critical bonuses stack?
- Is there a 'catch' at all with this feat?
The feat text says:
Benefit: When you use an arcane attack power and you are wielding a magic implement in each hand, you can add the off-hand implement’s enhancement bonus to damage rolls. Both of your implements must be usable with this power, and you must be capable of wielding both implements, to gain this benefit.
So the enhancement bonuses do stack. The text says "you can add the ... bonus", not "you can use the ... bonus".
Other properties I'd say don't stack, you choose to conduct the attack with one implement or the other.
The feat just increases the damage of an arcane attack power by the enhancement modifier of the off-hand implement. So all that this feat does is increase the damage output of the caster. Only the main-hand weapon is considered for any power-based interactions (crits, special power effects, etc).
One important thing to note about this feat is that each time you use the duel implement you can select which one is the main-hand and which is the off-hand. This becomes important if your caster is using superior implements (i.e. petrified orb or accurate staff) or when he has implements that interact with different powers he has.
A 'catch' that catches some casters is when they don't have an implement that allows them to make opportunity attack. If you have a wand and an orb, you can't take free swipes at guys. That's usually not too bad for casters, but free damage is free damage.
From the wording of the feat as Adriano posted it, it looks like only the damage part of the enhancement bonus for the offhand implement is added. So if you had a +2 Magic Wand in your main hand, and a +1 Magic Wand in your offhand, together they would give you a +2 enhancement bonus to hit, and a +3 enhancement bonus to damage.
Since only the damage addition is mentioned in the feat, the offhand implement would not add to critical damage or add any properties to spell used.
The enhancement bonuses of both your main hand and off hand implements are both added to damage. This is the purpose of the feat.
As for properties of the implements, it depends on the property. Normally, only one implement is used when using a power, and dual implement spellcaster does not change that. The implement through which you cast the power applies it's properties to the power (critical damage, saving throw modifiers, situational to hit or damage bonuses, and so on), however some implements have properties that come into effect regardless of the implement being used to cast the power.
A sorcerer using a rhythm blade dagger in his or her off hand, would gain the defensive bonus that the rhythm blade provides as that is a property not contingent on using the implement to cast a power.
http://wizards.custhelp.com/app/answers/detail/a_id/1396/~/d%26d%3A-faq-for-the-player%27s-handbook
clearly states that Warlocks can use properties of both implements but can only attack with one. that means the properties stack, but the criticals do not. If you have a jagged +3 in one hand, and a bloodiron +3 in the other...you attack with the bloodiron, but you still get the 19-20 crit range of the jagged. Thus when you crit on a 19, you do 3d10 dmg this turn and 3d10 at the start of your next.
How this applies to sorcerers? well, the consensus is that warlocks have nothing in their description that differs them from sorcerers with regards to implements. As such, the general agreement is that this applies to sorcerers. Obviously your DM could over rule this, but considering that sorcerer's tend to be considered underpowered against wizards (lack of options/etc), most let this go