I haven't managed to get Stadia to run properly with HW acceleration in a single browser on linux. Since my GTX 750Ti is pretty old and doesnt decode VP9 it gives me even more problems.
So I use WebCatalog https://webcatalog.io/webcatalog/
It's used to install webapps on your computer and Stadia is listed as one of them.
According to the arch wiki on hardware acceleration you may need to install libva,vdpau and some other packages to get proper HW decoding but the arch wiki will probably guide you through it better than me
Despite the marketing, Singlebox actually works with any web services, including Google ones.
You can also try WebCatalog. It has Spaces feature: https://webcatalog.io/webcatalog/spaces/. So basically, you can create multiple instances of Singlebox or Clovery and arrange them in different ways.