Hi 
I cannot see a workflow to do this as the event or condition does not allow you to look at “merge”. however you can do it out the box with the agent. 
when you merge there is an edit icon on the closed ticket you can click and then that will bring up a private note of which you can change to go back to the requester.

Hello @dobsonl, along with the suggestion from @afautley, you could look to trigger your workflow on the contents of the private note that gets added to the secondary ticket.

We have workflow that runs on the primary ticket of a merge but it could work the same way on the secondary ticket. Here is an example of the conditional we use for the primary ticket

And here is the trigger we use

Hope that helps!