At the other end OF Union Sq from El Potro is Cantina La Mexicana. Does this small area have enough room for two? I think so. La Mexicana feels a bit more upscale; the drinks are more sophisticated, and the list of tequilas and mezcals is impressive.
The nachos here are pretty exceptional. Fish main courses are ok, but the meat-based tacos, enchiladas and quesadillas are very satisfying. Definitely worth checking out.