When enabling the "volume replication" option for an existing logical volume (DSS V6 webgui -> CONFIGURATION -> volume manager -> Vol. replication -> vgxx -> volume manager) there will be a minimal downtime ( <1min), so the target will be inaccessible for ESX for a short period of time. If it's ok for you to stop the VM for about 2 minutes, you don't need to migrate the VMs to the second SAN. After you add "use volume replication" to a logical volume you can run the VM. Creating the volume replication task and starting the replication won't affect the VMs any more.

BR,
Tomek