Metal halide perovskites have emerged as promising photovoltaic materials, with record efficiencies beyond 20%. Among other, one of the unique properties of perovskites is their tunable band gap. While most studies have been directed towards the developments of small bandgap absorber, wide bandgap systems have not been explored in details. Realizing efficient solar cells with bandgap of about 2 eV would enable the fabrication of perovskite-perovskite tandem cells with efficiencies exceeding 23%, hence surpassing the efficiency of single junction devices. In this work we combine the beneficial stabilizing effects of Cs and FA cations to study the band gap variation and the properties of the perovskite compound Cs0.15FA0.85Pb(BrxI1-x)3 as a function of the halides composition, with the aim of developing an efficient complementary absorber for MAPbI3 in a perovskite tandem device. We have found the perovskite stoichiometry Cs0.15FA0.85Pb(Br0.7I0.3)3 to be a promising candidate, thanks to its band gap of approximately 2 eV. We also present an efficient monolithic tandem solar cells based on Cs0.15FA0.85Pb(BrxI1-x)3 and MAPbI3. By employing doped organic semiconductors, an efficient extraction of the photogenerated charge carriers is ensured, while carrier recombination at the perovskite interfaces is prohibited by using intrinsic and selective charge transport layers. Despite the non-ideal combination of bandgaps in the two subcells, we demonstrate perovskite-perovskite tandem cells delivering an average PCE of 15%, with a maximum efficiency obtained of 18%.