Based on the information below, which is the basic description of the Level 1 Power: Pact Weapon, I would say that you can use the Dual Implement Feat. However, it would only apply to the powers that include the weapon keyword unless you could figure out a way to have the weapon be considered an implement. I know this isn't exactly supported by the information below but...
My non-rules based opinion is: Why prevent your Hex-Blade from using the defining class feature of the Hex-Blade just because they want to be a dual wielding caster? Sure they save a little $$ by not needing to enchant two implements, however, if you disarm (or steal) the original implement from them they lose access to BOTH. Seems like a potential handicap to me.
Level 1: Pact Weapon
Your pact weapon is a physical manifestation of the arcane alliance you have struck with an otherworldly power.
Benefit: While you hold your implement in one hand, you can use a minor action to manifest your pact weapon in the other hand. The pact weapon you create depends on your pact (fey, page 225, or infernal, page 230). Your pact weapon persists until you no longer hold either it or the implement, or until you dismiss it as a free action.
You can make weapon attacks with your pact weapon, using its proficiency bonus and the appropriate damage die. Your pact weapon shares your implement’s enhancement bonus, critical hit effect, properties, and powers. The weapon cannot be enchanted.
When you use a power associated with your pact weapon and the power has both the weapon and the implement keyword, you are considered to be wielding both your pact weapon and your implement for the purpose of feats and other game elements.