Thanks for the suggestions, I think I know what the problem is, I have tested the install several times before from the hard disk and I changed the folder name to the name of the 2nd disk (NFSUDISK2) to no avail, but I found that if I edit the autorun.inf...:
Name=Need For Speed Underground
...and change Disk=1 to Disk=2 it immediately asks for CD1 so the installer is looking in the autorun.inf. I also tried Disk=1,2 and Disk=1 and Disk=2 below each other, also deleted the Disk=1 part completely and even removed the whole autorun.inf, but that won't work either.
Also there's an AutoRun folder with an autorun.cfg with this contents:
Registry=EA GAMES\Need For Speed Underground
AppName=Need For Speed Underground
Is there something I am missing which I can change, I enabled EASOInstall (dunno what that is), and tried the last three settings to no avail, I did change the standard language which works, so that means the installer does look in the autorun.cfg aswell.
If no-one nows how to edit the autorun files the right way I think I'm gonna burn two CDs since the guy I'm burning the DVD for has no experience with mounting images.