I'm running Reader 9.4.7 on Ubuntu 10.04 amd64. There are menu items on the Reader Window menu to cascade windows or tile them vertically or horizontally, but nothing happens when I select these menu items - I can still only see one of the tabbed open documents at a time. The same menu items work fine in the Windows version of Reader. Were the Adobe programmers just too lazy to either remove them or implement the code behind them when building the Unix version?
The only way to view two PDFs is to detach the tab for one of them into a completely separate window. That sort of works, but a fair bit of screen space is lost through having two separate Reader windows open, each with a tab, menu bar, and toolbar.