Microsoft has included a new feature in the latest build of Windows 10 and while it may not seem like a big deal right now, it paves the way for an improved user experience going forward. That feature is the ability to open multiple instances of a Modern app.
In Windows 10 build 10130, you can now open multiple instances of the calculator app. Right now, this is the only app that can support this feature but sources close to Microsoft who wish to remain anonymous say this feature is coming to more apps too. Not only first party apps, but third party ones too, will eventually gain this functionality.
Microsoft is also considering the creation of an API that would allow Modern apps to spawn new windows and/or deep link into other Modern apps.
The ability to run multiple instances of an app can be limited by the developer too. As you can imagine, Spotify and the like will only want their app to be run in a single instance scenario as multiple instances violates their terms of use, so this functionality can be controlled.
Our sources could not agree on how soon we will see this feature roll out to all apps, as different branches have different features, but they do all agree that it is coming to Windows 10.