Open Up Your Document
In photoshop i've opened up an image of charlize theron (just for this example).
Duplicating The Window
Now we have our image open lets duplicate the window, to do this we need to go to "window" in the menu bar.
Go to "arrange" then at the bottom click "new window for "file name here".
Providing your document window is not maximized you should have two windows. If it is maximized just restore it down.
Tiling The Document Window
If you head back into the "window" options under the arrange tab you have two options for tiling the document windows, horizontally and vertically.
Tiling Vertically
If you click "tile vertically" your windows will be displayed like this.
Tiling Horizontally
If you click "tile horizontally" your windows will be displayed like this.
Free Too Zoom
Each window can be zoomed and edited differently but the overall result will be applied to whole image. For example if i were to change her eye color.
The changes are made in the top window and its updated in the bottom window, notice the zoom levels of each window. If you would like the zoom level to be the same in both windows you can simply go to "window - arrange - match zoom level".
You can also match the windows location by going to "window - arrange - match location", there is also an option to do both at the same time by clicking "match zoom and location".
