You can only run ZP on one machine at a time. Been that way since the beginning. If you own Pro version you can install ZP on 3 computers, but only one can run the poster at a time. Project Maker can be opened by all at the same time.
FAQ:
Q: What does it mean «Count of computers» and how does it differ from «Running copies»?
A: Although you are authorized to install the Standard and Professional versions of the program on multiple computers, the program can only be executed on one computer at a time. ProjectMaker is an exception you can run it on all machines simultaneously.