How To Use The “New Window For” Command

In CS6 and newer, you can go into the Window Menu and select Arrange > New Window for [name of your document]; this will allow you to have the same document opened in 2 different windows. You can then go into Window  > Arrange > 2-up vertical to tile the 2 windows side-by-side. 

This is very useful when you want to both, work in an extreme zoom level, and also want to see the entire image as you make your adjustments. Any changes you make in one window will instantly be applied to the second.