Merge two contact records. The contact ID in the URL will be treated as the primary contact, and the contact ID in the request body will be treated as the secondary contact.  For more details about what happens when you merge two contact records, please see this knowledge article.

Note: Merging contacts cannot be undone. The data from the secondary contact will be merged into the primary contact, and while you can see the combined history of the two records in the resulting single contact, there is not a way to separate the contacts after the merge.