Nginx, Apache Prefork, Apache MPM, and PHP FPM all have multiple processes, and I would NEVER try to split up subprocesses of any of these into multiple containers.
It's quite simple if something communicates through shared memory, it should probably be in the same container. If it doesn't then that's NOT a reason to keep it in the the same container, but I don't see how it justifies the work of splitting it up....