How can I start an app across two monitors to create a wide display in linux

Hi, I am a newbie.
I want to start my game with wide view across my two 1920x1080 monitors in my Linux desktop (I want to open my game with 3840x1080). I increased ‘width’ to 3840 in ‘Display’ in game.project. But it does not cross boundary of primary display(Its width is not over 1920, remains in 1920).