Just found that Adobe just launched a preview of Adobe Browser Lab  a tool for cross-browser testing of web pages and apps  as it would appear on different browsers and operating systems,and  it let’s you define browser sets for testing and set 1-up view 2-up view or onion-skin view,  check Adobe Labs for more info or visit Adobe BrowserLab

BrowserLab provides web designers exact renderings of their web pages in multiple browsers and operating systems, on demand. BrowserLab is a powerful solution for cross-browser compatibility testing, featuring multiple viewing and comparison tools, as well as customizable preferences. Since BrowserLab is an online service, it can be accessed from virtually any computer connected to the web. Also, Adobe Dreamweaver® CS4 software users have access to additional functionality such as testing local and active content.