Dual GPU problem with multiple displays in GNU/Linux

That was it! Once I tried another - newer - hardware:

inxi -Fxz
System:
  Host: tv-mgmt Kernel: 4.15.0-54-generic x86_64 bits: 64 compiler: gcc 
  v: 7.4.0 Desktop: Xfce 4.12.3 Distro: Linux Mint 19.2 Tina 
  base: Ubuntu 18.04 bionic 
Machine:
  Type: Desktop System: Gigabyte product: B360 HD3 v: N/A serial: <filter> 
  Mobo: Gigabyte model: B360HD3 v: x.x serial: <filter> 
  UEFI [Legacy]: American Megatrends v: F13 date: 03/14/2019 
CPU:
  Topology: 6-Core model: Intel Core i5-8400 bits: 64 type: MCP 
  arch: Kaby Lake rev: A L2 cache: 9216 KiB 
  flags: lm nx pae sse sse2 sse3 sse4_1 sse4_2 ssse3 vmx bogomips: 33696 
  Speed: 800 MHz min/max: 800/4000 MHz Core speeds (MHz): 1: 800 2: 800 
  3: 800 4: 800 5: 800 6: 800 
Graphics:
  Device-1: NVIDIA vendor: Gigabyte driver: nvidia v: 435.21 bus ID: 01:00.0 
  Device-2: NVIDIA vendor: Gigabyte driver: nvidia v: 435.21 bus ID: 02:00.0 
  Display: x11 server: X.Org 1.19.6 driver: nvidia tty: N/A 
  OpenGL: renderer: GeForce GTX 1650/PCIe/SSE2 v: 4.6.0 NVIDIA 435.21 
  direct render: Yes 
Audio:
  Device-1: Intel Cannon Lake PCH cAVS vendor: Gigabyte 
  driver: snd_hda_intel v: kernel bus ID: 00:1f.3 
  Device-2: NVIDIA vendor: Gigabyte driver: snd_hda_intel v: kernel 
  bus ID: 01:00.1 
  Device-3: NVIDIA vendor: Gigabyte driver: snd_hda_intel v: kernel 
  bus ID: 02:00.1 
  Sound Server: ALSA v: k4.15.0-54-generic 
Network:
  Device-1: Realtek RTL8111/8168/8411 PCI Express Gigabit Ethernet 
  vendor: Gigabyte driver: r8169 v: 2.3LK-NAPI port: 3000 bus ID: 04:00.0 
  IF: enp4s0 state: up speed: 100 Mbps duplex: full mac: <filter> 
Drives:
  Local Storage: total: 312.51 GiB used: 7.29 GiB (2.3%) 
  ID-1: /dev/sda vendor: Western Digital model: WD3200AAKS-00B3A0 
  size: 298.09 GiB 
  ID-2: /dev/sdb type: USB vendor: Kingston model: DataTraveler 2.0 
  size: 14.42 GiB 
Partition:
  ID-1: / size: 291.46 GiB used: 7.23 GiB (2.5%) fs: ext4 dev: /dev/dm-0 
  ID-2: swap-1 size: 980.0 MiB used: 0 KiB (0.0%) fs: swap dev: /dev/dm-1 
Sensors:
  System Temperatures: cpu: 54.0 C mobo: 27.8 C gpu: nvidia temp: 46 C 
  Fan Speeds (RPM): N/A gpu: nvidia fan: 0% 
Info:
  Processes: 217 Uptime: 16h 52m Memory: 31.35 GiB used: 2.26 GiB (7.2%) 
  Init: systemd runlevel: 5 Compilers: gcc: 7.4.0 Shell: bash v: 4.4.20 
  inxi: 3.0.32

… it worked out of the box. (With Xinerama set up via the nvidia-settings GUI.)

Thank you very much for helping me out. :)