Could he have killed the girls before Shanann came home?
Shanann, although tired and worn out, would have been happy to finally arrive home to see the girls. What if he had killed the girls earlier, bagged and stored them in the basement, right below the basement stairs.
He then lies to Shanann that he had arranged the girls to stay at the Lindstrom's and they agreed take the girls to school. They have their backpacks packed with blankies and extra clothing for school. Tells her he loves her and to get some rest she deserves. So thankful to her thoughtful husband, she goes to sleep...