Names, Surnames & Titles

Keeping two names

Q:

1) When 2 names are kept, eg Faathimah Zahraa, then is it fine -or not advisable- to call by only one of the names at times. Eg calling the child sometimes by both names, sometimes only faathimah, sometimes zahraa, is this fine?

2) When calling by both names together, would it be pronounced Faathima-tuz-zahraa', or Faathimah zahraa', or would both be fine to say?