How to open two instances of the same file in Visual Studio or split horizontally
1) In your project just select the file you want to split horizontally and go to (Window->New Window), this will duplicate your current window instance.
2) Now you have two copies of the same opened file, so just press right mouse click button and select (New Vertical Tab Group) in one of them.
this will split into 2 views.
3) Result: As you can see now you have the same file duplicates and you can edit in any of them, all your work will duplicates automatically in both windows.