There is no way to do that. The replication factor (and search factor) apply to all buckets in an index that is being replicated.
Glad you got your question answered 🙂 Can you resolve the post by clicking "Accept" directly below @lguinn_splunk's answer please? Be sure to do this with all of your questions with a working answer. Thanks!
Just out of curiosity, what were you planning to set the retention periods (for hot/warm and cold)?