Enhanced Linked Mode with embedded PSC is not recommended, although can be done, but the second PSC must be joined to the same SSO domain.
Take a look here: VMware KB: VMware Platform Services Controller 6.0 FAQs
When should I use Embedded? When should I used External? What is the optimal PSC to vCenter Server Architecture?
vCenter Servers with Embedded Platform Services Controllers are designed for small environments in which no vCenter Servers, or 2nd party VMware products, need to communicate (via Enhanced Linked Mode) and the vSphere environment stays relatively static. In these environments, often there is only a single vCenter Server. When using the Embedded Platform Services Controller with the vCenter Server, it is not recommended to setup replication partnerships with External Platform Services controllers or other, embedded Platform Services Controllers. At this time, there is no way to reconfigure a vCenter Server with Embedded Platform Services Controller to an External Platform Services controller or vice versa.
External Platform Services Controllers are used in large environments in which multiple vCenter Servers are all working in conjunction via Enhanced Linked Mode and/or you have multiple 2nd-party applications (vRealize Automation Center, vRealize Orchestrator, etc.) that integrate with the PSC. In these environments, often there are multiple vCenter Servers connected to the same vSphere domain, and there are other 2nd-party applications that stack on top of the vCenter, using it as an endpoint for automation or Cloud services.
For more information, see List of recommended topologies for vSphere 6.0.x (2108548).
And here: How to properly architect and deploy vCenter 6.0 - Jonathan McDonald - VMware Blogs
In this case vCenter is installed as an embedded configuration on the first server. Subsequent installations are configured in embedded mode, but joined to an existing Single Sign-On domain.
Linking embedded Platform Services Controllers is possible, but is not a recommended configuration. It is preferred to have an external configuration for the Platform Services Controller.
The configuration looks like this:
