How can I tell Ubuntu to display using my 1050 Ti rather than my 1080 Ti?

I built a linux box with a GTX 1080 Ti, mainly for Machine Learning experiments, which has been working fine for about a year.

I am now adding a 4K screen, so I will have dual screen setup. In order to leave the 1080 Ti unburdened by driving screens, I added a GTX 1050 Ti just for that purpose.

However, at the moment I can’t get Ubuntu to use the 1050 for the displays; the screens stay dark.

The driver seems to work fine though, at least nvidia-smi lists both cards:

+-----------------------------------------------------------------------------+ | NVIDIA-SMI 430.40       Driver Version: 430.40       CUDA Version: 10.1     | |-------------------------------+----------------------+----------------------+ | GPU  Name        Persistence-M| Bus-Id        Disp.A | Volatile Uncorr. ECC | | Fan  Temp  Perf  Pwr:Usage/Cap|         Memory-Usage | GPU-Util  Compute M. | |===============================+======================+======================| |   0  GeForce GTX 108...  Off  | 00000000:01:00.0  On |                  N/A | |  0%   46C    P8    16W / 250W |    284MiB / 11169MiB |      0%      Default | +-------------------------------+----------------------+----------------------+ |   1  GeForce GTX 105...  Off  | 00000000:02:00.0 Off |                  N/A | | 29%   32C    P8    N/A /  75W |      2MiB /  4040MiB |      0%      Default | +-------------------------------+----------------------+----------------------+  +-----------------------------------------------------------------------------+ | Processes:                                                       GPU Memory | |  GPU       PID   Type   Process name                             Usage      | |=============================================================================| |    0      1464      G   /usr/lib/xorg/Xorg                            18MiB | |    0      1500      G   /usr/bin/gnome-shell                          49MiB | |    0      1808      G   /usr/lib/xorg/Xorg                           108MiB | |    0      1939      G   /usr/bin/gnome-shell                         100MiB | |    0      4332      G   nvidia-settings                                4MiB | +-----------------------------------------------------------------------------+ 

Any hints welcome!