Run Photoshop and Photoshop Beta together on Windows?

Actually Windows definitely allows running multiple instances of the same app, as described here and here. Pretty sure it's actually the default unless special handling is added.

So they must have chosen to only allow one instance, for one reason or another. Perhaps it's the way their dynamic linking stuff works between multiple apps, maybe on Windows it would act up with multiple instances.

For some apps you can get around this if you create a 2nd Windows user account, if you Shift + Right Click and hit "Run as a Different User", but it seems even this doesn't work with Photoshop.

However you can do it using sandboxing software like "Sandboxie Plus" and it's App Compartment feature. With that you can run the Beta and another version, multiple instances of the same one, etc.

Though when running the 2nd copy within Sandboxie, it asks you to login to creative cloud so it probably would count as your 2nd "computer" license I guess. There might be a way to configure it in a way to not isolate it that much though.