The powerblades are attached to the wrists, or even at the elbows, while the kiss is placed over the Harlequins hand. Also, the blades don't have to be on the arm, I put my models blades sticking out of his knees and feet, to show that he doesn't just fight with his hands.
And if you mean from a rules perspective, it specificaly says that powerblades can be used with 2 CC weapons for +2 attacks. Also, no model may have more than one two handed weapon and one one handed weapon, or two one handed weapons. Since powerblades are wargear, they don't use a choice.