A topic much like this question is being discussed here: One online player lockes out server CPU for one core (forge)
As far as I can tell there are rumours that some java tweaks, using the correct java version, that will work on some servers to force them multicore.
Core usage is more of an java thing than a server thing, since it is the java engine that enables multicore if suported. I guess you’d have to program the server jars in a way that utilizes multiple cores to be able to make use of more than one core effectively.