I have two monitors attached to a 980. Just got the second one recently. One is an Acer and one is a Viewsonic. Both are 2560x1440 gsyncs.
When I start DOS enhanced or DOS 2 the Viewsonic flickers then goes blank and image is still on the Acer. When I start DOS plain they both show the game normally.
I think the Acer is hooked up via HDMI and the Viewsonic with DisplayPort if that is pertinent.
And Windows 10 is set to show same image on both monitors cuz I use one standing and one sitting. Haven't had any other issues in the 4 days I have been using the Viewsonic.