Apple - Two page continuous view in Lion Preview?
This is one of those things that really makes me wonder "If it's not broken why fix it?".
Other than switching to another application that might give you back the lost functionality, I only have one suggestion at the moment. This is how I've been able to use Preview (5.5) and still get two page (side by side) continuous scrolling:
Open up your pdf, document, etc. in Preview.app
File > Print or ⌘P
In the Print Dialog Box click Preview > Change to Layout [ 2 pages ]
Lastly, click PDF > Open PDF in Preview.
You should now see your document layout as two pages and you can scroll continuously. I realize it's far from ideal, although it is a way to still use Preview exclusively, yet still have similar functionality as before. I hope they will fix it; I shouldn't have to be thinking of ways like this to use it like it could have been. ;-)
The free app Skim supports continuous-scroll side-by-side pages. To use that view, select the menu item PDF → PDF Display → Two Pages Continuous.
Also, if the left-hand page is wrongly displayed on the right side and vice versa, you can fix that by toggling Book Mode in the same menu.