There is no merge option in the Documents menu when I have selected two documents in the Binder view.
This may well not be implemented yet - I know Split at Selection was only implemented about two days ago. I’ll leave it for Lee to confirm though - he’s asleep in Australia at the moment. 
Sleeping? Who told him he could do that? 
If it’s not implemented yet, it’s probably best to highlight the text in the next beta tutorial.
Thanks!