Note: The Merge and Delete feature is only available for the Salesforce CRM.
While we can't merge two records directly in Outreach, Outreach will sync any merged records in Salesforce. This requires the Plugin to be connected to Salesforce. Also, the Merge and Delete feature is enabled. If the instance is not connected and Syncing to Salesforce, you will have to delete one of the records manually from inside of Outreach.
For Dynamics, you need to merge the records in Dynamics. Keep the master record in Outreach, and delete the other record. Make sure to merge into the master record that you're planning to keep in Outreach. Outreach should update the information after the merge as usual.
There is no mechanism to merge records at all within Outreach.