thank you very much for reply and sorry for confusion. Of course you are right: I want blog1.domain1, blog2.domain1.. and city1.domain2, city2.domain2...
That this can be done with the multi-domains plugin is great news for me.
Just to clearify:
This also means, that I can have to different main-/parent-sites: domain1 and domain2, where domain1 can e.g. list all the blogs 1-... and domain2 can e.g. list all the cities 1-... Correct?
Sorry for my silly questions