Image disappears when same layer is picked for both sides

[jfrankl]

When I switch both sides of the slider to the same layer, one side disappears. Then, when switching the other layer picker, the application switches displays image on that side, removing it from the first.

Functionally, there's not a good reason to select the same layer on both sides. But in practice, when you're toggling around to check out the app, it can happen pretty easily.