I have a laptop (Core i7 Haswell, intergrated graphics) with a HDMI and VGA outs. I have the laptop screen and monitors attached to both outputs. Both monitors are capable of displaying at 2560x1440. Unfortunately KDE freezes after setting one monitor to 2560x1440 when both laptop outputs are on.

**Edit**
It appears stable if I boot without VGA device connected, set HDMI device to desired resolution. Connect VGA device.

How do you setup the screen? krandr, kscreen or xrandr?
In doubt, try xrandr (commandline tool, very direct and post the command you used (if it still triggers the issue)
Also check if only compositing is affected (Shift+Alt+F12 toggles the compositor)

In that case please provide outputs of

xrandr -q
qdbus org.kde.kwin /KWin supportInformation
glxinfo -l

Given your comments concerning the order of connection and resolution change, this could quite possibly be a driver issue. In this case, it would be helpful if you can post the content of /var/log/Xorg.0.log.old after you have rebooted the system.