You can leverage extended vMotion (new in vSphere 5.1) that does not require shared storage.
So you could move one host from vCenter A to Center B, live migrate VMs from it to some other hosts (using x-vMotion) and vice versa,
then move the host back to vCenter A.
However, if you are using dVS then this would require extra steps (migrating networking to standard switches and back to dVS).