Hello, to start, I use 2 monitors. The first one is a 4k monitor which supports up to 240hz, and the other one is a 1440p monitor which supports up to 165hz. When I set 165hz on the 1440p monitor, everything works flawlessly. But when I try to set 240hz on the 4k monitor, everything turns into chaos. The 4k monitor goes black with no signal, and the 1440p monitor starts flickering, until the settings revert after 15 seconds, so on the 4K monitor I am stuck on 120hz max. I should also mention that I use wayland, kde plasma, and my video card is an RTX 4070 Ti. I would really like to continue using wayland, as scaling to me is very important. I also would like to somehow, if possible, set the 4K monitor to work on 240hz. Please help!
If you read https://www.onecomputerguy.com/165hz-vs-240hz/ You may decide that 165Hz is totally fine. Of course that s just onecomputerguy s opinion 
The 4K monitor doesn’t support 165hz, only the 1440p monitor does. The 4K monitor supports: 60hz 120hz and 240hz, while the 1440p monitor only supports 60hz, 120hz, and 165hz
and you are trying to feed it 240hz. There lies a conundrum. That s probably why it s flickering…The way I see it is either get another video card to support the 2nd monitor, or another monitor that supports 240hz, or 165hz; whichever you prefer.
Oh. I think I read your initial post wrong 
I think you meant that you only set the 4k monitor to 240hz right? Yea. Hmm, I would ask how you have the monitors connected to the computer. Are both connected to a video card, or maybe one is to a vid card, and the other is connected directly to the motherboard? Also, how do you adjust the refresh rates? xrandr seems to be a way to do it. Also it could be how you have them connected. I read that display port is better for high refresh rates than hdmi. Try here,
Sorry for the late response. Yes, you are completely right. I switched to GNOME but the same problem persists. I use the settings to manage my refresh rates. My both monitors are connected to the video card using DisplayPort. Here is the xrandr -q output:
Screen 0: minimum 16 x 16, current 6400 x 2160, maximum 32767 x 32767
DP-2 connected 2560x1440+3840+0 (normal left inverted right x axis y axis) 600mm x 340mm
2560x1440 164.47*+
1920x1440 164.45
1600x1200 164.50
1440x1080 164.46
1400x1050 164.47
1280x1024 164.31
1280x960 164.43
1152x864 164.28
1024x768 164.37
800x600 164.28
640x480 163.71
320x240 163.69
1920x1200 164.50
1680x1050 164.50
1440x900 164.40
1280x800 164.52
1152x720 164.38
960x600 164.40
928x580 164.20
800x500 164.25
768x480 164.02
720x480 164.18
640x400 164.01
320x200 161.83
2048x1152 164.48
1920x1080 164.50
1600x900 164.48
1368x768 164.35
1280x720 164.38
1024x576 164.37
864x486 164.31
720x400 163.64
640x350 163.70
DP-1 connected primary 3840x2160+0+0 (normal left inverted right x axis y axis) 700mm x 400mm
3840x2160 119.98*+
2048x1536 119.95
1920x1440 119.97
1600x1200 119.82
1440x1080 119.92
1400x1050 119.90
1280x1024 119.83
1280x960 119.89
1152x864 119.77
1024x768 119.80
800x600 119.85
640x480 119.52
320x240 117.34
2560x1600 119.93
1920x1200 119.90
1680x1050 119.89
1440x900 119.94
1280x800 119.85
1152x720 119.73
960x600 119.74
928x580 119.55
800x500 119.47
768x480 119.71
720x480 119.65
640x400 119.64
320x200 117.55
3200x1800 119.96
2880x1620 119.95
2560x1440 119.95
2048x1152 119.96
1920x1080 119.93
1600x900 119.95
1368x768 119.83
1280x720 119.86
1024x576 119.85
864x486 119.69
720x400 119.54
640x350 119.24
I m trying to think logically about what to check, but in reality I ve never used 2 monitors, so my experience is nill in that regard. I have ran across several threads discussing running multiple monitors at different refresh rates though. It seems like it s commonly done. One thing I do have experience with is a bad cable. I was trying to get my 4k tv to run at 120hz, and it would not until I replaced the cable which was the problem, so taking a systematic approach seems appropriate in your situation. In your case it could be a myriad of things. Off the top of my head, it could be as simple as replacing a cable, as I mentioned previously, changing the aspect ratio/s, or enabling fractional scaling on one or both monitors. Doing a quick search I found a few sites that may help narrow down the problem, or at least give new ideas as how to solve your problem.
This thread is for Ubuntu, but can maybe help get the troubleshooting juices flowing. It interestingly mentions fractional scaling as how the problem was solved. One could look into that avenue.
and a link from same thread:
and of course archlinux wiki
https://wiki.archlinux.org/title/Xrandr
Good luck 
i question my self, isnt that like : https://wiki.archlinux.org/title/Variable_refresh_rate#Wayland_configuration
?
Hello, sorry for late reply, but I recently switched to Hyprland and somehow 240hz now works. Thanks for the tips tho, I might use them in the future, who knows 
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