I would imagine that there will no "cloning" and that both spanned and independent display devices will be able to take advantage of showing the background. Otherwise what's the point?
Dream Aquarium flip-flops the background on the second monitor.
Marine Aquarium 3 actually has a continuous background which is nearly 3 times as wide as a normal monitor, of which just part of it will be seen on 1 or even 2 monitors. Conceivably on 3 monitors, if you have a powerful enough video card, you would see the entire background and get black bars at the left of monitor #1 and the right of monitor #3.