integrated graphic card as primary monitor + nvidia gtx 1080ti for hashcat

hi folks, im having some trouble on configuring nvidia-settings in xorg. i have i7 8700k and 2x gtx 1080tis.
i would like to use integrated graphic card of i7 8700k as primary monitor where i can read and do my research stuff, 2 1080ti should only do gpu computing. Ubuntu 16.04 is the os, nvidia 390 driver is insalled. in bios integrated graphic card is enabled (igpu) , everything works fine. i have primary monitor and 2x 1080ti do hashcat on 100%, accept i cant set --cool-bits=28 in nvidia-settings which lets me overclocking and fan speed regulation of both cards. my xorg.conf gets overwritten after every boot to this:

Section "ServerLayout"
Identifier "layout"
Screen 0 "nvidia"
Inactive "intel"
EndSection

Section "Device"
Identifier "intel"
Driver "modesetting"
BusID "PCI:0@0:2:0"
Option "AccelMethod" "None"
EndSection

Section "Screen"
Identifier "intel"
Device "intel"
EndSection

Section "Device"
Identifier "nvidia"
Driver "nvidia"
BusID "PCI:1@0:0:0"
Option "ConstrainCursor" "off"
EndSection

Section "Screen"
Identifier "nvidia"
Device "nvidia"
Option "AllowEmptyInitialConfiguration" "on"
Option "IgnoreDisplayDevices" "CRT"
EndSection

Section "Device"
Identifier "nvidia"
Driver "nvidia"
BusID "PCI:2@0:0:0"
Option "ConstrainCursor" "off"
EndSection

Section "Screen"
Identifier "nvidia"
Device "nvidia"
Option "AllowEmptyInitialConfiguration" "on"
Option "IgnoreDisplayDevices" "CRT"


nvidia-settings -a "[gpu:0]/GPUFanControlState=1" -a "[fan:0]/GPUTargetFanSpeed=50" gives me:
ERROR: Error assigning value 50 to attribute 'GPUTargetFanSpeed' (men:0[fan:0]) as specified in assignment '[fan:0]/GPUTargetFanSpeed=50' (Unknown Error).

lspci | grep VGA

00:02.0 VGA compatible controller: Intel Corporation Device 3e92
01:00.0 VGA compatible controller: NVIDIA Corporation GP102 [GeForce GTX 1080 Ti] (rev a1)
02:00.0 VGA compatible controller: NVIDIA Corporation GP102 [GeForce GTX 1080 Ti] (rev a1)

nvidia-smi
Tue Sep 11 16:06:36 2018
+-----------------------------------------------------------------------------+
| NVIDIA-SMI 390.87 Driver Version: 390.87 |
|-------------------------------+----------------------+----------------------+
| 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 Off | N/A |
| 31% 54C P0 62W / 250W | 226MiB / 11178MiB | 0% Default |
+-------------------------------+----------------------+----------------------+
| 1 GeForce GTX 108... Off | 00000000:02:00.0 Off | N/A |
| 23% 29C P8 11W / 250W | 2MiB / 11178MiB | 0% Default |
+-------------------------------+----------------------+----------------------+

+-----------------------------------------------------------------------------+
| Processes: GPU Memory |
| GPU PID Type Process name Usage |
|=============================================================================|
| 0 1136 G /usr/lib/xorg/Xorg 224MiB |
+-----------------------------------------------------------------------------+


So i cant regulate fan speed of both 1080tis or overclock them. Any help?

thanks