Do they let the "CPU" run for a couple cycles, then let the "SPC" run for one cycle, and repeat?
More like SPC is run from one CPU access to the next which would be much more effective, with occasional timeout catchups to prevent too large processing spikes and be ready with sound for say the current frame/end of sound buffer or something. SPC cannot really do anything to outside world outside sound mute signal so the emulation can pretty much sit idle and only run when external is happening, at least that's how I would do it.