How to span a window between two displays in Mavericks

[iMore.com]

If you use OS X Mavericks with multiple displays, you may have noticed a change in the system's behavior since you upgraded: You now get docks and menus in each window. But there's a drawback: you can't display a single window between multiple displays anymore. Either the app window is on one display or the other, but never both. Here's a handy tip for restoring that functionality, for anyone who misses it from Mountain Lion.

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