For communication that stays on same host you don't need certificate. We don't know what is your use case.
When running ES+Kibana on same host, the purpose of elasticsearch's certificate may be to secure connection from remote data shippers to ES. The purpose of the certificate in Kibana would be to present your Kibana server to public and to encipher data between client's browser and Kibana.
You can use same certificate for both of them, especially while just testing.