that would just be over complicating things, the important thing is that the two machines should never be running on the same host. the rules as they are prevent that.
if you need greater granularity then get your client to upgrade to 5.x where HA events interrogate DRS before starting up to conform that they comply with DRS rules.
currently the situation is that if a vMotion event attempts to move a guest in contravention of a DRS rule vMotion will move the other machine to another host before completing the move.
as a machine is not fully moved from machine A to machine B until vMotion is complete this satisfies the DRS Rule.