Mexican restaurants seem to be on a bit of a roll lately, with second and third locations of popular spots popping up around town. A second outpost of Olecito, the tiny yet notable take-out sister to Olé Mexican Grill in Cambridge, has opened in Brookline (700 Commonwealth Ave.; 617-353-7257). And Tu Y Yo, an authentic Mexican restaurant in Somerville, recently debuted a second outpost in Needham (66 Chestnut St.; 781-453-1000).