How do I transfer icloud email from one apple id to another

If you mean transfer the actual @icloud.com address, you can't: addresses, whether the primary address or aliases, can't be moved from one account to another. If you mean just transferring messages, in the Mail application on your Mac you can drag them from iCloud to 'On My Mac' and then back up to the other iCloud account. If there are a lot it will take some time.