Hi Wilson,
If I understand your question correctly, you have two separate CallManager servers in two separate clusters and you wish to consolidate them into a single cluster, correct?
If that is the question, then the answer is there's no way to demote a publisher to have it become a subscriber in an existing cluster. You will need to install it from scratch as a subscriber in the existing cluster.
You will therefore have some work to do around recreating all the devices currently configured in the publisher that will be decommissioned and configuring them in the other cluster. The Bulk Administration Tool may come in handy here.
Another consideration here includes making sure that the requirements for clustering across a WAN are met.
Hope this helps.
Regards,
Michael.