Leo I and Leo II are the two most remote dSph galaxies satellites of the Milky Way, situated at about 250 Kpc of it. They are objects of similar size and low metallicity, but their star formation histories appear to be fundamentally different. The very deep HST color-magnitude diagrams (CMDs) of these galaxies reach the position of the oldest turnoffs, and allow to immediately conclude that Leo I has a large amount of intermediate-age population, while Leo II is predominantly old. The quantitative study we are performing using synthetic CMDs is allowing us to retrieve the details of the star formation histories of these galaxies, with a resolution in time of about 1--2 Gyr. We will show the results obtained for the star formation histories of both galaxies, and present a comparative discussion that will allow us to gain some insight about the conditions that lead to such different star formation histories in otherwise very similar dwarf galaxies.