Floyd's questions about wattage
No, you still don't have it right.
A double sided 1 cube/day screen would be 3x4 and would need 12W of CFL split between both sides (6W per side)
A single sided 1 cube/day screen would be 6x4 and would need 12W of CFL all on one side. If you had 2 3x4 screens, then each screen would need 6W on one side.
Wattage stays the same.
The physical surface area of growth substrate stays the same. On a double-sided screen, you actually have 24 sq in of growth area. For a single sided screen to have the same capacity, you need to have the same growth area, but since you are only using one side, the dimensional area of the screen must double.
The reason the screen sizing guideline isn't geared around physical growth area (surface area) is that it gets confusing. It's easier to just talk about length x width and leave it at that.