Right-click or control-click a note that isn't already open.
Click "Open In New Window" from the menu that appears.
Tips and Tricks
To switch between windows, use the "command+`" keyboard shortcut. (In case you're not familiar with this shortcut, the "`" key is to the left of the "1" key.)
To switch to the window for a specific note, click "Window" in Notability's menu bar and click the desired note from the list that appears.
When using Multi-Note on Mac, your Library will only be available in one window. To switch to that window, click "Window > Library" in Notability's menu bar or use the "option+command+L" keyboard shortcut.