Would adding a VM-Host Affinity Rule help at all?
Perhaps adding a rule that allows VM1 to only be hosted on hosts 1 through 4 and VM2 on hosts 5 through 8? My thought on that being that if you manually, or in a HA event for example, try and move VM2 to one of the first 4 ESXi hosts, it would fail, and in turn not invoke a vMotion of VM1.
That might be over complicating the whole thing though. I've sure the client wouldn't notice a vMotion on the "live" server too.