Not definitively, but what I was able to surmise is that although they are both in the same SSO domain (by name), which is the very common vsphere.local, the SSO Site Names are different, and there is no way that I could find to change this.
In my case I wanted to do it because I have an old vCenter that I wanted to migrate off of as seamlessly as possible. Since I can't, I am using vSphere Replication 6.1 to move the VM's from one to the other.