There are a headache inducing number of threads that never actually resolved this issue on the wizards site. Not linked because I feel dumber after reading them.
Can a hexblade use dual implement spellcaster given that they conjure their weapon from their implement and they have become proficient with their conjured weapon as an implement through multiclassing?
Best Answer
Yes, a pact blade implementified, dual implement spellcasting hexblade double dips on enhancement bouns damage
Dual Implement Spellcaster'sDDI buff requires that you wield a magic implement in each hand, that both implements be usable with the power, and that you must be capable of wielding both implements.
If we assume that by some means light blades have been learned as an implement type (for example via swordmage multiclass) then a fey-pact hexblade could wield their Blade of Winter's MourningDDI as an implement.
Since manifesting the Blade of Winter's Mourning requires that the hexblade hold an implement in one hand, the hexblade now has an implement they are capable of wielding in either hand. And as long as the power being used is (a) an implement power, and (b) not an implement power for another class gained in a way such that the "usable with the power" clause is not violated all the requirements of Dual Implement Spellcaster are met.
The only question remaining is does the Pact Weapon class feature somehow deny this combination. The only relevant passage appears to be:
You could argue that since the pact weapon shares the implement's enhancement bonus, it can't stack with the implement, but since a double weapon's enhancement bonus can be made to stack with it's self I don't see a problem here.