Well, today I tried again with a current kubuntu trusty and I cannot get the secondary card working whatever I try. With 13.10 it works fine... If someone would like to help me figure it out here are the relevant logs:

http://pastebin.com/hEuE8SZE working dmesg (13.10)
http://pastebin.com/WeSByULi working Xorg.0.log

http://pastebin.com/wSSH3r91 bad dmesg (14.04)
http://pastebin.com/dTrgDnAd bad Xorg.0.log

As you can see, Xorg doesn't load nouveau on the second card anymore. I'd report a bug, but I don't know exactly against which component...

Thanks