Moin, moin,
I am using openSUSE 15.5 on an old intel based x86 PC, equipped with a simple
nvidia gforce and two monitors. The monitors are using a displayport
connnection from the graphics card to the monitors via an Y-shaped cable. Both
monitors are recognised correctly and can be configured to form a single
graphical work space, when the system is up and running.  

After upgrading to openSUSE 15.6 this does not work any more. When booting,
monitor 1 shows some firmware screen and the openSUSE boot screen, where the OS
version (several subversions of 15.6 and frozen snapshots of 15.5) can be
selected. Using the default option (15.6 from harddrive), the output of the
boot process is moving to monitor 2, but monitor 1 is going to a "power saving
sleep mode" and stays inactive frome here on. The boot process is finished
correctly with monitor 2 as the only graphical device. Monitor 1 cannot be
activated again.

Switching off monitor 2 before booting, the system comes up without any valid
output device. Monitor 1 goes to sleep after the system boot started. I did not
check the system status with a remote ssh-connection yet. 

Switching off monitor 1 before booting, lets the system come up completely with
monitor 2. Also the initial hardware screen is diplayerd there. 

In turn, booting from a read-only snapshot with openSUSE 15.5, both monitors
are used as before. The nouveau-dri drivers are used, no propriatary nvidia
driver is installed. 

Some upgrade history:
- just to test, if the few graphic card cores could be used in a cuda
environment, cuda was installed under 15.5. This failed and cuda was
deinstalled. However, when upgrading later from 15.5 to 15.6 the cuda
development environment was reinstalled automatically together with nvidia
drivers. As a result the nvidia drivers allowed only for a single low
resolution vga monitor. May be, this happened, since the nvidia repository was
not deleted and there was some leftover from the cuda test. 
- with the ascii-yast the nvidia drivers (gforce003/004) and cuda could be
removed and the nouveau-dri drivers where reinstalled - together with
dependencies. This allows for a single monitor OS_15.6 system with high
resolution, but the second monitor (monitor 1) is lost during the boot process
as described above.

Before I try to install from scratch just with the hope that the error goes
away, I am reporting this strange behaviour here. May be, this helps to
streamline the distribution. Any hints are very welcome - especially how to
modify the bootloader options.
Cheers,
Martin Schmidt