I have focusrite 6i6, second gen. If you cant play several instances at same time, it must be because you did not choose asio either in your amp sim or computer sound. I do not have any problem playing spotify, vlc, youtube, daw, standalone, and all does thing played at once. Make sure your amp sim has chosen asio, and focusrite asio driver, and that your computer have asio chosen. And check if you might have any update for that asio driver from focusrite web page. They often update their software.
Look in my post in that link on how to optimize Pc for audio on Windows. There should be a link to a guide…