"linked" windows works well Rated 5 out of 5 stars
I've grown accustomed to placing 2 windows side-by-side. This add-on automates keeping the windows side-by-side; you can drag width independently and height simultaneously. All the bars and links are available in the active window so you can open new tabs on, or close either window.
I think this is a great improvement over manually viewing and adjusting 2 windows at-a-tiime and I'm not sure that viewing 2 pages in one tab would work as well.