When a merge takes place the redundant profile is archived (made inactive) with the reason of Merged. 

On the redundant profile, some of the details remain, such as the Profile details and ChangeLogs, but any Timeline information is passed to the constituent profile that it was merged into. 

Where a merge has taken place and needs to be undone, the archived profile can be reinstated if it has not been deleted by other means (e.g a bulk delete). If deleted, you would need to create a new profile.

Once un-archived, it's going to be a task of identifying each timeline item and reassigning each one that needs to be moved manually to the re-activated/new profile.

The Change Log on the Timeline may be of assistance with this part of the task.

To reassign an item, you will need to open it and then click on the Manage button, and then choose the reassign option. It will then ask for the constituent that you are reassigning to. Once reassigned it will show on the other constituent Timeline instead.

For items that can not be reassigned, these will need to be added manually to the correct Constituent Timeline and then deleted from the incorrect Constituent Timeline.