Merge the active tab and the previous one into a frameset
Once you click at the extension's icon, the current tab will be merged with the previous one (on the left) into a frameset with two columns (frames) or rows (options offer 3 buttons: rows/columns/ad_hoc). They'll be ordered in the same way as the tabs (left, right).
In other words, your physical screen will be split into two areas.
If the current window only has one tab, you'll be asked about the URL of the left frame. Recursive frames work - e.g. if you activate the extension on a new tab - but each smaller frame's URL has to be typed manually ("same origin policy").
This gadget may be helpful if you are e.g. translating one page to another page or otherwise editing a text, using two windows.
Don't test the extension on this gallery of extensions or other "https" pages. Use ordinary "http" pages only; the "scheme" has to agree. Also, some pages such as "TraceMyIP.org" deliberately use a "framekiller" to jump out of the frames; I know of no way how to overwrite their stubborn opinions.
*Analogous add-on on Firefox*
ChromeSplits, Split screen, Chrome screen splitter
*Other Chrome extensions by the same developer*
*A brief history of the extension*
Version 1.1: Options: vertical/horizontal frames
Version 1.2: Now, the active plus the left from it are merged
Version 1.3: One or two missing URLs asked
Version 1.4: New tab opened near the active one
Version 1.5: New option: ask me every time columns/rows (default)
Version 1.6: Recursive frames allowed, unknown URLs entered via forms