Mixed cation perovskites are currently the most efficient perovskite materials used in perovskite solar cells. Mixing two cations inside a perovskite structure results in enhanced flexibility when designing interesting material properties. Moreover, using two inorganic cations in the same perovskite maintains the advantage of fully inorganic structures. A fascinating subject to investigate is therefore the nanoscale synthesis and the properties of such mixed inorganic cation perovskites. In this work we mixed Rb and Cs inorganic atoms inside perovskite nanoparticles. We explored down to the atomic resolution different Rb and Cs concentrations and performed the chemical mapping of single nanoparticles. At medium concentrations, the Rb atoms are observed in the core of the particles, whereas the Cs atoms are located in the shell region, forming core shell structures. However, if there are high concentrations of Rb, a phase separation occurs because bulk perovskite based solely on Rb cations is not stable at room temperature. Density functional theory calculations support our experimental observations by showing that a stable nanoparticle is formed when the Rb atoms are located inside the particle and not on the surface. Our work demonstrates the importance of understanding the perovskite structure at the atomic level, leading to the formation of mixed cation bulk perovskites and nanoparticles, and to improved perovskite stability. A new phase of cesium lead bromide (Cs6Pb5Br16) related to the Rb6Pb5Cl16 structure is also reported.