Supported in the Paperspace native app. Click View > Add Display. This is currently supported on Windows Stanard GPU machines, but not the Dedicated GPU machines at this time. Linux support is planned.
While you can enable the second monitor in your web browser, there are some limitations to multi-monitor display on browsers due to inherent limitations in web browser technology.
To enable multi-monitor on your Paperspace machine through your browser window, click the Paperspace Menu, choose Advanced Settings, and toggle the Multi-monitor switch on. This enables multi-monitor display, so when you click out of that window and click the Menu again, you will see multi-monitor is now an option. Selecting it from the drop-down will connect the second window to your display.