I think you could get by pretty well with only one semester of organic. The organic section focuses on a lot of concepts, rather than individual reactions, and you learn the majority of these concepts during the first semester. However, I would definitely recommend studying or at least reading about some major second semester reactions/concepts. These include the properties of aromatic compounds, reactions of aldehydes and ketones, aldol condensation reactions, electrophilic addition reactions, and NMR/IR/Mass spectroscopy. These were the major topics covered in my second semester of organic, and many showed up on my MCAT.
