The performance of a tandem solar cell depends on the performance of its constituting subcells. Although this dependency is theoretically straightforward for open-circuit voltage (V-oc) and short-circuit current, it is indirect for fill factor (FF) and thus for efficiency. We study here with simple simulations the effect on the tandem performance of each subcell FF by varying systematically their series resistance, parallel resistance, and local defect. We demonstrate that series resistance impacts strongly FF for single-junction devices but marginally for tandem devices, the opposite holding for parallel resistances (shunting). We show that localized defects will be most stringent to the tandem device when they occur in the current-limiting subcell. There is thus no obvious correlation between FFs of a tandem device and of its subcells. Finally, we compare two bottom-cell designs and highlight the importance of using a high-V-oc bottom cell to reach high tandem efficiencies.