After attaching my second monitor, I can open display properties and select and enable it. After pressing apply, Xorg crashes and puts me back to the login screen. The monitor then can be used. A suspend and resume cycle crashes X again regardless of the monitor still being attached or not. Re-attaching sometimes also immediately crashes X. Setting the computer to sleep with the monitor attached sometimes keeps the computer flashing the suspend led until I hard reset it. The logfile after one such crash is attached. It gives the error "(EE) NVIDIA(0): Failed to allocate primary buffer: out of memory." which I found some related bug reports to but none in ubuntu that crashes X. There is a similar bug report for the exact same machine (Thinkpad R61) without solution for Mageia here: https://bugs.mageia.org/show_bug.cgi?id=9229#c13.
Just to confirm, the same computer monitor combination can be used fine in windows, so it should be capable of the combined resolution.
(The same behaviour might also be seen using the nouveau driver, but I have to try that again).
Another error case is as follows: successfully use the second monitor after cold starting, disable it in display properties. Suspend and later resume the system without the monitor attached. Attach the monitor and enable it in display properties, Xorg will crash.
After logging in again, trying to activate the monitor will not result in a crash but will give the following error message (in my locale, german):
GDBus.Error: