Seeing a File Twice in Visual Studio
You want to compare/view two parts of the same file at the same time. You have two choices for making this happen.
First, you can click on the divider bar at the top of the scroll bar on the right side of your editor window. Dragging that bar down divides your code window into two panes (one on top of the other), which you can scroll through independently.
However, each of these panes is now smaller than the original window. If you want to see your code in a full-size window then go to the Window menu and select New Window. You'll get a second window with the same name but with ":1" tacked on the end (for example, "Customer.cs" and "Customer.cs:1"). If you want, you can drag one of the tabs to another monitor (something you can't do with the divider bar).
Either way, changes you make in one window/pane automatically appear in the other window/pane.
If you're using Visual Studio 2010, this feature is turned off for some languages (the team ran out of time to completely test it). You'll need to make a change to your Windows Registry to turn it on.
Posted by Peter Vogel on 01/14/2016