Zoomaround
Zoom'n
Now this clarifies it for me. Thanks again, Cap'n. It's been a while since I read the book so I don't remember that he actually had a sword and then a knife. You're right, the film doesn't really show the scene in it's entirety.
The editing in the film is poorly done at that scene. What happens is this: the WK comes out with drawn sword, then as he assails Frodo he pulls out his Morgul knife, sword in the other hand (there is also some reversing going on). At some point that the editing does not fully show, he sheaths his sword, leaving only the knife. It's kind of hastily done, so it's pretty confusing no matter how you view it. I hope that helps at least a little.