[RPG] Dual Implement Spellcaster and double weapons

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.

Yes, a pact blade implementified, dual implement spellcasting hexblade double dips on enhancement bouns damage

Dual Implement Spellcaster's^DDI 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 Mourning^DDI 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:

Your pact weapon shares your implement's enhancement bonus, critical hit effect, properties, and powers.

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.