Is it possible to build planetary fortress and orbital command in one command center?
I didn't think is possible, but I saw some threads that tell build OC and then PF...
Am I correctly understand that it is possible to build both of them?
No, it's not possible (Wish it were, that would rule.)
People who build PF usually do it as a cheap way to defend an early expansion. But OC is better to start because your start should be able to defend itself at least as well as a PF could do it, and having an early detector can be key.