I currently have VCenter 5.5 (3252642) installed at 2 sites. The entire system is on a single server at both sites. I wish to upgrade to Vcenter 6 (ISO 3040890). We will be installing Replicator and SRM and have been told and seen documentation that the recommendation is to use separate Platform Services servers and to have the 2 sites in enhanced linked mode. I need documentation on how get from 5.5 single server to 6.0 with separate Platform Services. I know this cannot be done during the upgrade but can find no documents explaining the process to get where I'm going.
** Introduced in vSphere 6.0 Update 1, you can decommission the Embedded Platform Services Controller on the vCenter Server and repoint the vCenter Server to an External Platform Service Controller. You must upgrade the vCenter Server and Platform Services Controllers to Update 1 in order to use this process.
so, answer to your question would be, Upgrade your vCenter 5.5 to vCenter 6 Update 1, that will be embedded PSC during upgrade.
now if you deploy a separate host with only PSC and do Enhanced Linked Mode with your existing PSC, let it SSO replicate.
and follow the process to repoint existing your vCenter 6.0 update 1 to external PSC.
Link to upgrade guide
npadmani - You are partially correct. The problem with your process is that you'll end up with 2 vCenters with embedded PSCs that have 2 unique SSO domains. In 6.0 we do not have a way to merge SSO domains or move a vCenter from one SSO domain to another.
So, in order to end up with 2 vCenters in the same SSO domain, first the OP needs to get the 2 vCenters to a common SSO domain prior to moving to 6.0. To do this deploy a new 5.5 external SSO instance at each site and then re-register the 2 vCenters to the new SSO instances. Remove the embedded instances once everything is verified to be working. Refer to VMware KB: Migrating embedded VMware vCenter Single Sign-On to an external VMware vCenter Single Sig...
Next, upgrade the external SSO instances to 6.0 PSCs. Finally, upgrade the vCenters to 6.0.