Hello Christian,
It seems that this is a known issue with Microsoft Loop PWA. According to a post on the Microsoft Community forum, there is no way to open multiple windows of Loop PWA anymore. However, you can try using window.open() to create “child” windows. When you call the function, it’ll return a Window object which you can interact with. The child window can access the parent by accessing window.opener. Now both windows can see each other.
Hope this helps!
-Fathia