I have GeForce GT 1030 card with 2 Dell U2415 monitors. The connection is 1030's DisplayPort => Monitor 1 DP Input; Monitor 1 DP Output => Monitor 2 DP Input.
The OS is Debian Buster (10), with the default nouveau driver.
I have tried all possible scenarios I can think of w/o any success. What's in common is that when I boot the machine both screens are showing the BIOS/GRUB/OS booting. The action comes when Xorg comes.
Nouveau driver, regular boot. The login screen is seen on the 2nd screen, after login I can see only xfce4 background and the mouse can leave to the left where the main (1st) screen should be, but it is black. Xorg.log
Nouveau driver, nomodeset boot. Both monitors have blinking underscore with Xorg dead. Xorg.log
Nvidia driver (from the Debian's non-free repo). xfce4-display-settings shows the 2 monitors, but the 2nd one is marked as 'Disabled'. If I enable the 2nd screen, the screen on the 1st one blinks and both are seen as active. The mouse can leave the 1st monitor to where the screen of the 2nd should be, but the 2nd is totally black. Selecting 'mirror displays' doesn't help either - still black screen on the 2nd monitor. The 'configure new displays when connected' is checked. Tried to turn off the 2nd screen - it dissapears from both xfce4-display-settings and nvidia X server settings. Turning it on again asks me how to deal with the monitor. Selecting to extend to the right accepts my change, both monitors are seen as active on both xfce4-display-settings and nvidia X server settings. The position of the 2nd one is Absolute +1920+0 as it should be, but it is still black. Xorg.log
Same Nvidia driver, with the xorg.conf provided by the Nvidia X server settings xfce4-display-settings pops up with Unable to start the XFCE display settings. Unable to query the version of the RandR extension being used. xrandr command returns RandR extension missing. Only first screen works, the 2nd one is black Xorg.log
The monitors are tested with the same cables from one Intel NUK using the same OS (Debian 10 Buster) but with the intel's graphic card and the Xinerama works properly out of the box.
What I'm missing which can be done in order to feed the 2 monitors using the GT 1030?