Multi-monitor display is available in the native app and web browser by selecting "Add Monitor" from the Paperspace menu while inside your VM, which will launch a secondary display window for your VM.
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.
Limitations
Mutimonitor is not supported on Dedicated GPU machines at the moment (GPU+,P4000,P5000,P6000,V100). Vertical secondary monitors have some display/sizing issues.