The last part of the description reads, "or sometimes one blade like that of a katar."
To me, the orc knuckle dagger sounds like a trench knife or a push dagger depending on your preference. Which matches the image just below the item description on page 286.
The second blade there is much smaller, probably a back-up if you don't have enough time to reverse your "grip" and bring the primary blade back into action. You'd likely only cut yourself if you you kept thumbing the edge.
