based on given examples a failure of ems1 will make streams unavailable on other ems, too as they are chained.
Byside of this design issue, you have to monitor the "health & usage" of your ressources and as well define how user get assigned to which ems for playback. If this is done in a good way, you only need to create the aliases on the EMS which get allocated for the user playback.
Of course, if you would setup a cluster of EMS you need to sync the alises over all nodes inside the cluster. But this looks not fitting, as you mention the EMS are "fare away", so I believe on different Continents ?