Martin,
So you are using the same PERNR for both usernames. Only thing I can think of is after switching the username on the PERNR try running PFUD and then also when you login as the same PERNR but different users are you able to see the team members in the Team Page for both users? also in that same page try the Refresh button and then see if it changes anything for you in the Team Calendar view. Also compare the USERS parameters in SU01.
Hope one of those suggestions will help.
Thanks,
Martha.