If you use kafka firehose, then due to current limitations described in http://druid.io/docs/latest/ingestion/overview.html#ingest-from-apache-kafka , you can either do partitioning or replication but not both. If you need both then you would have to use tranquility with indexing service.
Now, I guess, you have 2 realtime nodes with different kafka consumer groups. That means, both are receiving and publishing same data. You would indicate that to druid by setting the “shardSpec” correctly , see druid.io/docs/latest/ingestion/realtime-ingestion.html#sharding .
In your case, both realtime nodes would set the shardSpec to…
same partition numbers indicates to druid that they same and at query time only one is queried.