How can I force Plasma 5 to treat two monitors as one in Kubuntu 16.04?

Question

Goal

I want to use KDE across two monitors, but I only want the window manager to see them as one single virtual monitor. I would like to achieve this goal either using KDE settings, nvidia-settings, or xorg.conf.

What I've tried

I have tried enabling Xinerama in my xorg.conf file and it works at the login page, but then the Plasma desktop doesn't load. I get no taskbar and have to use alt-F2 to run anything, as well as encounter lots of other problems. I have also tried TwinView as I am running nvidia, but I get the same above problems.

More info

Basically I want the taskbar to stretch across both screens, and to be able to maximize windows to take up both screens. I've had this problem in every flavor of Ubuntu (Ubuntu 15, Kubuntu, Mint 17) and every window manager (Openbox, Compiz, Awesome) I've tested.

Here is a screenshot of what I see:

xorg.conf

# nvidia-settings: X configuration file generated by nvidia-settings
# nvidia-settings:  version 361.42  (buildd@lgw01-18)  Tue Apr  5 14:33:28 UTC 2016

Section "ServerLayout"
    Identifier     "Layout0"
    Screen      0  "Screen0" 0 0
    InputDevice    "Keyboard0" "CoreKeyboard"
    InputDevice    "Mouse0" "CorePointer"
    Option         "Xinerama" "0"
    Option         "Twinview"   "true"
EndSection

Section "Files"
EndSection

Section "Module"
    Load           "dbe"
    Load           "extmod"
    Load           "type1"
    Load           "freetype"
    Load           "glx"
EndSection

Section "InputDevice"
    # generated from default
    Identifier     "Mouse0"
    Driver         "mouse"
    Option         "Protocol" "auto"
    Option         "Device" "/dev/psaux"
    Option         "Emulate3Buttons" "no"
    Option         "ZAxisMapping" "4 5"
EndSection

Section "InputDevice"
    # generated from default
    Identifier     "Keyboard0"
    Driver         "kbd"
EndSection

Section "Monitor"
    # HorizSync source: edid, VertRefresh source: edid
    Identifier     "Monitor0"
    VendorName     "Unknown"
    ModelName      "DELL UP2414Q"
    HorizSync       31.0 - 140.0
    VertRefresh     29.0 - 75.0
    Option         "DPMS"
EndSection

Section "Device"
    Identifier     "Device0"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce GTX 970"
EndSection

Section "Screen"
    Identifier     "Screen0"
    Device         "Device0"
    Monitor        "Monitor0"
    DefaultDepth    24
    Option         "nvidiaXineramaInfoOrder" "DFP-2.9"
    Option         "metamodes" "DP-0.8: nvidia-auto-select +1920+0, DP-0.9: nvidia-auto-select +0+0"    
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection

Other window managers might work better, but I really like the one in Plasma 5 and would prefer to keep using it.

Answer 1

Turn off Twinview. Change this line in xorg.conf

   Option         "Twinview"   "false"

Note: When you maximize a window it will go across both monitors, but I think this is what you want.