I have just installed devuan, which installs by default with XFCE, then plasma-desktop to get KDE.

I have an xorg.conf generated with X -configure, which contains both (identical) monitors, both (identical) graphics cards, proper screen and server layout sections and uses the appropriate PCI IDs.

When I start an X session using XFCE, everything works fine, and I can use the second monitor. When I start it using KDE, the second monitor is always black, I can't drag windows onto it, and the cursor is a black cross. There's just one monitor available in KDE Settings - Display.

Why doesn't KDE use both monitors? How can I tell it to use the second too?

Parts from /var/log/Xorg.0.log which I consider relevant - that tell me that X itself does make use of both monitors/graphics cards, just KDE does not (but I may misinterpret the log, I don't know much about X):

xorg.conf loaded:

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[    10.416] (==) ServerLayout "X.org Configured"
[    10.416] (**) |-->Screen "Screen1" (0)
[    10.416] (**) |   |-->Monitor "Monitor1"
[    10.424] (**) |   |-->Device "Card1"
[    10.424] (**) |-->Screen "Screen0" (1)
[    10.424] (**) |   |-->Monitor "Monitor0"
[    10.424] (**) |   |-->Device "Card0"
[    10.424] (**) |-->Input Device "Mouse0"
[    10.424] (**) |-->Input Device "Keyboard0"

second card:

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[    10.531] (II) NOUVEAU(0): Output DVI-I-1 using monitor section Monitor1
[    10.532] (II) NOUVEAU(0): Output HDMI-1 has no monitor section
[    10.560] (II) NOUVEAU(0): Output VGA-1 has no monitor section
[    10.560] (II) NOUVEAU(0): 3 crtcs needed for screen.
[    10.562] (II) NOUVEAU(0): Allocated crtc nr. 0 to this screen.
[    10.562] (II) NOUVEAU(0): Allocated crtc nr. 1 to this screen.
[    10.573] (II) NOUVEAU(0): EDID for output DVI-I-1
[    10.574] (II) NOUVEAU(0): EDID for output HDMI-1
[    10.602] (II) NOUVEAU(0): EDID for output VGA-1
[    10.602] (II) NOUVEAU(0): Manufacturer: SAM  Model: b42  Serial#: 808662328
[    10.602] (II) NOUVEAU(0): Year: 2014  Week: 43
[    10.602] (II) NOUVEAU(0): EDID Version: 1.3
[    10.602] (II) NOUVEAU(0): Analog Display Input,  Input Voltage Level: 0.700/0.300 V

first card:

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[    10.659] (II) NOUVEAU(1): Output DVI-I-2 using monitor section Monitor0
[    10.660] (II) NOUVEAU(1): Output HDMI-2 has no monitor section
[    10.670] (II) NOUVEAU(1): Output VGA-2 has no monitor section
[    10.670] (II) NOUVEAU(1): 3 crtcs needed for screen.
[    10.672] (II) NOUVEAU(1): Allocated crtc nr. 0 to this screen.
[    10.672] (II) NOUVEAU(1): Allocated crtc nr. 1 to this screen.
[    10.726] (II) NOUVEAU(1): EDID for output DVI-I-2
[    10.726] (II) NOUVEAU(1): Manufacturer: SAM  Model: b43  Serial#: 808662328
[    10.726] (II) NOUVEAU(1): Year: 2014  Week: 43
[    10.726] (II) NOUVEAU(1): EDID Version: 1.3
[    10.726] (II) NOUVEAU(1): Digital Display Input

I have no idea why it logs about Card1 before Card0, but I assume it's benign and due to the order it gets the cards in from the kernel. Card0 has PCI ID 2:0:0 and Card1 has PCI ID 7:0:0 in xorg.conf.

I had the same hardware running properly with KDE before, only with the nvidia proprietary driver, not with nouveau. On devuan the proprietary driver does not compile (out of the box) due to some regression bug in the driver, it seems. I'd like to get it running with the nouveau driver, though.

Update: I now have two monitors in use, after having installed the nvidia proprietary driver (a newer revision without the kernel module build problems), and almost everything seems to work fine.

Only, when maximizing windows, they maximize over both screens, which is undesirable. How can I prevent this?

With xinerama enabled, and the proprietary nvidia driver, under System settings -> Display there's nothing except an error message: No kscreen backend found. Please check your kscreen installation.

It has to be a KDE issue, since with xfce maximization happens to only one screen - which is the reason why I don't post the xorg.conf or the X startup log, no errors or exotic settings there anyway.

Have you got kscreen installed? If yes, purge and reinstall. With any luck the error message will disappear.