2 monitors - problem

[MrG8]

Hey! I just wanted to ask why there is a problem with the in-game resolution when you're using 2 monitors.
I don't play in windowed mode but I still want the other screen to see what's going on at other sites. In other games that works fine. But in WOW I can't set the resolution to 1920x1080 and 144hz.
But If I deactivate the 2 monitors and only play at one monitor it works find setting those settings.

What is the problem?

[MrG8]

I think that even if I don't use windowed mode it still only let me set the settings on what screen number 2 can take. Not the screen it using