This article explores the intertwined urban and cultural histories of Buenos Aires, Argentina, and Montevideo, Uruguay during the early twentieth century. During the years from 1910 to 1936, Buenos Aires and Montevideo’s relationship evolved from being “mirrored” and “symbiotic” into an uneven relationship as Buenos Aires’s skyline and cultural commerce expanded to compare to larger metropolitan centers in the Atlantic world. This article examines how the relationship between the two cities was understood by flâneurs, travel writers, and cultural producers in the crucial period of metropolitan growth and cultural commerce for both Buenos Aires and Montevideo. The cultural connections between the two cities transpired in a wider transnational and transatlantic context of urban capitalism that spurred the construction of modern urban spaces such as skyscrapers, theaters, and department stores. The relationship between Buenos Aires and Montevideo was mirrored by neighboring and competing metropolises across the globe in the early twentieth century.