When you saying PSC in same site, it means under the same logical object of Site name you chose during installation.
What are Sites in PSC 6.0?
A Site in the VMware Directory Service is a logical container in which we group the Platform Services Controllers' server objects within a vSphere Domain. You can name them in an intuitive way for easier implementation. Additionally, when Platform Services Controllers are deployed, they publish their service information (service registrations) into the defined Site. When vCenter Servers are deployed against the Platform Services Controllers, the vCenter Server will publish its service information into the Site in which the Platform Services Controller belongs. If you need to move vCenter Servers between Site, you must move their respective service information. For more information, see the section Can I repoint the vCenter Server to other PSCs in the same vSphere Domain? Can I repoint the vCenter Server to a new vSphere Domain? within this article.
For you case, have a look on this KB Reconfigure a Standalone vCenter Server with an Embedded Platform Services Controller to a vCenter Server with an Extern… You can re point this new vCenter to existing external PSC controller. But note that repointing option is available with 6.0 Update 1, not with 6.0 GA and 6.5 https://haveyoutriedreinstalling.com/2016/11/18/vsphere-6-5-repointing-woes/#content
All possible PSC topologies : List of recommended topologies for VMware vSphere 6.0 (2108548) | VMware KB