My setup is the following: Ubuntu 13.10, i7-4770K, nvidia geforce 210. I have the HDMI port of the Intel chip connected to a 4K monitor (3840x2160) and the dual-link DVI output of the GeForce connected to a 2560x1600 monitor. By default both displays are recognized and enabled in landscape mode (with some weird mouse flickering), however physically the 2560 monitor is in portrait mode. When I use the "Screen display" UI or xrandr to rotate the display, X crashes and I'm sent to the login screen.
X log can be found here: http://paste.ubuntu.com/6753901/
I'm currently using the nouveau drivers, when I tried to use the proprietary ones I couldn't get the nvidia connected display to be recognized at all.