There are a couple of ways to work when you have more than one document open. One way is to have a tabs interface. This is the default document mode. This is easier if you need to quickly see what you have open. You can also discard documents quickly.
The other display type is subwindows. This allows you to view multiple documents at the same time. You can have the same document open with different zoom settings like the following image.
Artwork by David Revoy
Subwindow mode allows you to see the same document in multiple ways.
The shortcut Ctrl + Tab will switch to the next document. You can also press Ctrl + Shift + Tab to cycle backward through documents.
By default, Krita displays the tabs version for multiple documents. To change this, go through the main menu Settings > Configure Krita. On the General section, look at the Multiple Document Mode setting. When you change the setting and click OK, Krita will update its user interface to the new type.