From the fact that there is only one entry in the conf file of Kibana for ES cluster I doubt one Kibana instance can support more than one ES cluster endpoints but I am just wondering if it can support?
Elastic provides the tribe node feature that allows you to configure an elasticsearch client to connect to several elasticsearch clusters at once. You can set your kibana instance to connect to this elasticsearch client thus connecting to all clusters and viewing their indices in the same place. Note that this feature is deprecated in latest elastic version, and you should use the new feature called cross cluster search which is currently in beta mode.