Dual-phase steels are widely used in industrial sheet forming due to their high strength–ductility balance, but their complex microstructural response under load poses challenges for accurate process simulation. Additionally, the similar lattice structures of ferrite and martensite lead to strongly overlapping peaks in diffraction experiments. In this work, we present in-situ measurements, where lattice strains from synchrotron diffraction were combined with global strain data to capture the elasto-plastic behavior of a dual-phase steel under tension and compression. The peak separation was performed using the phase fractions as a fit constraint. Furthermore, the results were validated using the CMWP method. The experimental insights provide the basis for the calibration of a representative volume element (RVE) within a crystal plasticity finite element method (CPFEM) framework. The calibrated RVE allows us to link microstructural mechanisms to macroscopic response, and thereby to assess the transferability of the identified material parameters into large-scale sheet metal forming simulations. Our study demonstrates how synchrotron-based experiments can support the calibration of physically informed CPFEM models that improve the predictive capability of industrial forming process simulations.