Tandem solar cells, in which two individual cells are stacked on top of each other, offer the potential to increase the efficiency significantly compared to a single cell on the same area. To reach maximum efficiency, each cell in the stack must have a distinctive spectral response and the current in each cell must be similar. This requires smart selection of materials, proper cell design and appropriate layer thickness. Tandem polymer solar cells can be made by processing two individual cells from solvent based liquids, separated by a recombination layer. Potential candidates for the recombination layer are 1) a combination of a ZnO layer and a pH-neutral PEDOT:PSS layer, 2) a TiOx layer combined with a normal PEDOT:PSS layer. We will discuss the properties of the suggested recombination layers. To determine the performance of tandem cells, accurate spectral response measurements are crucial. Spectral response measurements of a polymer tandem cell show that the response of each subcell can be measured only when a bias light with sufficient intensity and suitable spectrum is applied. We will discuss the special requirements for the spectral response set-up that are needed in order to successfully discriminate between the responses of each subcell.