Hi Dirk,
I haven't been able to reproduce your problem. Here's what I did:
1- took a blank Moodle 1.8 install, added the face-to-face module
2- created a new course "course101"
3- created a new meta course "meta1" which has "course101" as a child course and uses the topics format
4- created two user accounts: "teacher" and "teacher2"
5- from within the meta1 course, I assigned the teacher role to "teacher" and "teacher2"
6- inside meta1, both teachers can create face-to-face activities and sessions
Is there anything different with your setup here?
Francois