I did solve it (but don´t ask how long it took)
I changed /etc/lightdm/lightdm.conf
cat /etc/lightdm/lightdm.conf | egrep -v '#'
cat /etc/lightdm/lightdm.conf | egrep -v '#'
[LightDM]
logind-check-graphical = True
run-directory=/run/lightdm
[Seat:*]
greeter-session=lightdm-gtk-greeter
session-wrapper=/etc/lightdm/Xsession
[Seat:seat0]
xserver-command=/usr/bin/X :0 vt7
xserver-layout=seat0
[Seat:seat1]
xserver-command=/usr/bin/X :1 vt8
xserver-layout=seat1
[XDMCPServer]
[VNCServer]
I reconfigured loginctl
loginctl seat-status seat1
loginctl seat-status seat1
seat1
Sessions: *4
Devices:
├─/sys/devices/pci0000:00/0000:00:01.1/0000:01:00.0/drm/card0
│ [MASTER] drm:card0
│ ├─/sys/devices/pci0000:00/0000:00:01.1/0000:01:00.0/drm/card0/card0-DVI-D-1
│ │ [MASTER] drm:card0-DVI-D-1
│ ├─/sys/devices/pci0000:00/0000:00:01.1/0000:01:00.0/drm/card0/card0-DVI-I-1
│ │ [MASTER] drm:card0-DVI-I-1
│ └─/sys/devices/pci0000:00/0000:00:01.1/0000:01:00.0/drm/card0/card0-HDMI-A-1
│ [MASTER] drm:card0-HDMI-A-1
├─/sys/devices/pci0000:00/0000:00:01.1/0000:01:00.0/graphics/fb1
│ graphics:fb1 "nouveau"
├─/sys/devices/pci0000:00/0000:00:01.1/0000:01:00.1/sound/card0
│ sound:card0 "NVidia_1"
│ ├─/sys/devices/pci0000:00/0000:00:01.1/0000:01:00.1/sound/card0/input20
│ │ input:input20 "HDA NVidia HDMI/DP,pcm=3"
│ ├─/sys/devices/pci0000:00/0000:00:01.1/0000:01:00.1/sound/card0/input21
│ │ input:input21 "HDA NVidia HDMI/DP,pcm=7"
│ ├─/sys/devices/pci0000:00/0000:00:01.1/0000:01:00.1/sound/card0/input22
│ │ input:input22 "HDA NVidia HDMI/DP,pcm=8"
│ ├─/sys/devices/pci0000:00/0000:00:01.1/0000:01:00.1/sound/card0/input23
│ │ input:input23 "HDA NVidia HDMI/DP,pcm=9"
│ ├─/sys/devices/pci0000:00/0000:00:01.1/0000:01:00.1/sound/card0/input24
│ │ input:input24 "HDA NVidia HDMI/DP,pcm=10"
│ └─/sys/devices/pci0000:00/0000:00:01.1/0000:01:00.1/sound/card0/input25
│ input:input25 "HDA NVidia HDMI/DP,pcm=11"
└─/sys/devices/pci0000:00/0000:00:01.3/0000:03:00.0/usb1/1-7/1-7.1
usb:1-7.1
├─/sys/devices/pci0000:00/0000:00:01.3/0000:03:00.0/usb1/1-7/1-7.1/1-7.1:1.2/0003:046D:C52B.0003/0003:046D:1024.0005/input/input17
│ input:input17 "Logitech M310"
└─/sys/devices/pci0000:00/0000:00:01.3/0000:03:00.0/usb1/1-7/1-7.1/1-7.1:1.2/0003:046D:C52B.0003/0003:046D:2011.0006/input/input16
input:input16 "Logitech K520"
├─/sys/devices/pci0000:00/0000:00:01.3/0000:03:00.0/usb1/1-7/1-7.1/1-7.1:1.2/0003:046D:C52B.0003/0003:046D:2011.0006/input/input16/inp>
│ leds:input16::capslock
├─/sys/devices/pci0000:00/0000:00:01.3/0000:03:00.0/usb1/1-7/1-7.1/1-7.1:1.2/0003:046D:C52B.0003/0003:046D:2011.0006/input/input16/inp>
│ leds:input16::compose
├─/sys/devices/pci0000:00/0000:00:01.3/0000:03:00.0/usb1/1-7/1-7.1/1-7.1:1.2/0003:046D:C52B.0003/0003:046D:2011.0006/input/input16/inp>
│ leds:input16::kana
├─/sys/devices/pci0000:00/0000:00:01.3/0000:03:00.0/usb1/1-7/1-7.1/1-7.1:1.2/0003:046D:C52B.0003/0003:046D:2011.0006/input/input16/inp>
│ leds:input16::numlock
└─/sys/devices/pci0000:00/0000:00:01.3/0000:03:00.0/usb1/1-7/1-7.1/1-7.1:1.2/0003:046D:C52B.0003/0003:046D:2011.0006/input/input16/inp>
leds:input16::scrolllock
Now i do have multiseat again
In the last few years i did work without any xorg.conf. This leads to xorg using internal defaults. This worked very good. (mostly 
)
- I only had to configure loginctrl as shown in the Arch wiki xorg multiseat with loginmanager
I had thought it would now be necessary to create again a xorg.conf because of my problems. But it does work without 
One problem remains
- When the computer boots, the monitor at seat0 shows
lightdm loginand works flawless. - The monitor at seat1 stays dark.
loginctl list-sessionssays there is alightdmat seat1 waiting for login
but the screen stays dark. - When the password for the user of seat1 is typed blindly into its keyboard (as if lightdm would have been visible), after the Enter key is pressed, the monitor of seat1 will suddenly show the desktop as if nothing has been wrong at all.
