I have similar situation in one of my projects and we use one PSC for two VC
just remember, because of dependencies between VC and PSC services, you should keep your PSC very High available. (VCHA, FT, Site Replication or any availability solution you can, provide it for PSC)
If there is a little for your PSC, both vCenter will be interrupted.
As others said, VMware highly recommended use Embedded PSC after release of vSphere 6.7, because all of the abilities that External model has, now embedded have them too