Sometimes you may want to see the same canvas on two displays.
This is useful in a couple of cases:
Work on the canvas on display A, but see a full screen version of the canvas on display B
Work on the full screen version on display A, but show the canvas on display B so that you can access the rest of the application's UI.
Some applications support this generic capability of putting the canvas on two displays simultaneously, but not all do.
Here are the ones I have confirmed that do support it: Clip Studio Paint, Krita, and Photoshop.
Let's say you have a document window already open called <title>
Click Window > Canvas > New Window
A new window showing the canvas for <title will appear>
Drag that window to another display
Now any changes you make in one canvas will be reflected in the other
Let's say you have a document window already open called <title>
Click Window > New Window
This creates a second new Krita application window that will have no documents in it.
Go to the original Krita application window and Click Window > New View <title>
Now you will see the document <title> on both application windows.
Then you can draw on that document in either application window.
Any changes to the document in one window will be reflected in the other.
If you have a document window already open called <title>
Use Window > Arrange > New Window for <title>
Now you will have two windows for the same document
Drag either window to a second display and maximize the window.
Then you can use either window to edit the document.