And there are two!

Well, Mrs. Owl finally got off the brood long enough for us to notice that a second hatching occurred today sometime between 4:49pm and 8:17pm. All the intervening time, she was sitting in the way of our view. In the first photo, you can see the two fuzzy shapes (one above, and one to the right of the remaining egg.)

In the second photo, we have zoomed in on the face of one of the owlets looking at the camera. Below the word "Face", you can just make out the owl-like features (eyes and bill are a bit darker than the rest of it.)
This is what a newly-hatched screech owlet looks like.