That isn't the same thing at all. This is a standard capability that, for example, is available in all office applications. Word, WordPro, and WordPerfect can all do it. It's an easy way to visually compare two documents. The other browsers let you open a second page, but for some unclear reasons, the second page appears in a second copy of the program.
Visual comparisons are more difficult.
And you can't use the standard ctrl-Tab to switch between pages. Opera comes with JavaScript, but not Java. To add Java, you must install a plug-in from Sun that's larger than the browser itself. The advantage is that you get the current version of Java. The disadvantage is it must be separately installed.