Improvement of solar cell performance through the parallel homo-tandem architecture is studied. The cell geometry is optically designed to maximize absorption of the AM1.5 spectrum. We consider both ITO electrode and a recently devised ITO-free alternative, the Two-Resonance Tapping Cavity. The optimal geometry strongly depends on this choice. We identify two mechanisms by which tandem cells outperform single cells: (i) back-up absorption and (ii) interferometric gain mediated by the central metallic electrode. We numerically demonstrate large improvement in short-circuit current for a wide choice of realistic materials: PTB7-th:PO71BM, P3HT:PC61BM, DBP:C-70, and methylammonium lead halide perovskite, as well as a theoretical material with a Shockley-Queiser type of spectral absorption plus an Urbach tail. Such an idealized response is found to reliably model real materials. A general response pattern is found that fits all the cases covered in our study, and which can serve as a guide to optimize homo-tandem cells.