What are they separated by? If it is something they can see through I would coax them both near the gate or barrier and have them facing each other so they can see one another then give them treats. This will show them that good things happen when they are in close proximity of each other. Once they get good at that do the same thng with no barrier. Have another person around to help you in case they do fight but if treats are involved they probably will be more focused on you
How long have you had Nala? It took us about a week or so before Gizmo was completely okay with our mixed puppy when we brought her home, that was over year ago.
Good Luck!