I have four cards here, one 1080 and three 1080Ti. When I first got the machine, the default graphics card was one 1080Ti. X server is also running on this 1080Ti.
The following steps explain how to set 1080 as the default graphics card.
Run the command:
to check the Bus ID address of the graphics card you want to set as the default.
From the above image, you can see that the Bus interface of the 1080 card is xxxxxx:02.x. You don't need to worry about the other 0s.
Run the commands:
sudo vim xorg.conf
Add a line "BusID" below the "Device" section, specifying the interface of the graphics card you want to set as the default.
Save, shut down, connect the monitor to the new default graphics card, and start up. Done~
Of course, these are just general steps. I encountered some problems when I did it. Don't worry, restarting or reinstalling Nvidia Drivers will always solve the problem.