Supported in the Paperspace native app. Click View > Add Display. This is currently supported on Windows VMs, but not the 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.