We are in the same boat. We do have a shared storage between two datacenters however there is no way you register the same VM under both of them. So what we have done in the past or even now if needed we just clone an image from one side to the shared storage and then import it on another vCenter then modify it, rename, repoint AV agent at local AVM  etc so that it is treated as new image. I'm pretty sure you could get very creative with vRom automation for that purpose but we don't have that available