I am designing a web app with a modal popup that has 3 steps. The first step contains two panes and occupies the full extent of the modal space (delineated by red dotted line) while the other two each occupy half or slightly more than half. Upon completion of each step, it slides to the left. It's not possible to make panes C and D fill the whole space because there just isn't enough content for that. I want a little bit of the previous and/or next panes visible.
Design 1 pros: Because each pane affects the next, you really should be able to see the next pane until you've confirmed your choices.
Design 1 cons: Not sure if that big spacing looks right.
Design 2 pros: Potentially looks better.
Design 2 cons: You'll have to see pane D while still editing C even though D is heavily dependent upon what you enter on C.