You can check, if your graphics card is able to handle 8K (2x4K) by opening a terminal and issuing the following command:
It will display some data like: (excerpt from my PC)
Screen 0: minimum 8 x 8, current 4480 x 1440, maximum 16384 x 16384
DVI-I-0 disconnected (normal left inverted right x axis y axis)
DVI-I-1 connected primary 1920x1080+0+360 (normal left inverted right x axis y axis) 598mm x 336mm..
The first line shows your max resolution, in my case it is
maximum 16384 x 16384
Since both displays work, you should have two Monitors connected (In the example you can see that "DVI-I-1" is connected. And it will show the possible resolutions for each monitor:
DVI-D-0 connected 2560x1440+1920+0 (normal left inverted right x axis y axis) 597mm x 336mm
2560x1440 59.95*+ 69.93
1920x1080 60.00 59.94 50.00 60.05 60.00 50.04
1280x720 60.00 59.94 50.00
1024x768 75.03 60.00
800x600 75.00 60.32
640x480 75.00 59.94 59.93
This shows, that my second monitor is connected as DVI-D-0, its highest resolution is 2560x1440, (framerate is 69.93 hz) but the entries below could be selected as well.