From http://www.vmware.com/files/pdf/vsphere_pricing.pdf (page 7)
vSphere Essentials and Essentials Plus Kits are self-contained solutions and may not be decoupled, or combined with other vSphere editions.
This means that except for the ROBO Kits you may only manage the hosts included in a kit with the also included vCenter Server.
Regarding your initial question, do you have two hosts in each site or two hosts all together? With only two hosts (up to 3 hosts) and enough bandwidth for replication, you could run replication with a single vCenter Server, there's no need for a second instance.
André