Supreme Commander did it all the way back in 2007, you could have a second independent viewport on the second monitor to keep an eye on a different part of the

Supreme Commander did it all the way back in 2007, you could have a second independent viewport on the second monitor to keep an eye on a different part of the map. IIRC it had to jump through some weird hoops to pull that off with the graphics APIs of the time, it spawned a second instance of the game which acted as a spectator client for the primary instance.