Each user can open 1 virtual workstation at a time, so if you need to open 2 virtual Workstation at the same time you need to do it with 2 diferent users.

If you are going to open both virtual Workstation from the same device you can for example:
Open de SimpleCloud desktop App connect to the virtual Workstation  and without closing the virtual Workstation  session, you can log out your user from the desktop App and log in with a diferent user.
Once logged in you can connect to the second virtual Workstation