Yes, it would probably be best to have the person with the best internet connection/best machine be the one who iniates a Paratext Live session. Then others can join that session as clients.
Regarding “primary editor”: Whoever starts the session is starting a Paratext Live server on their machine, so that machine would be the “primary editor”: the one that receives and processes all messages sent by clients (Machine A).
However, the machine referred to as the “primary editor” (Machine A) could be different than the person who is doing most of the editing (Machine B). So, you CAN have a different person doing most of the editing.
If the person doing most of the editing (Machine B) has a sub-optimal internet connection / sub-optimal machine, it would probably still be best to have the person with the best internet connection/best machine (Machine A) start the server, because Machine A is processing all of the traffic.