A biaxial device developed at DiffAbs beamline of the SOLEIL synchrotron facility has been employed to determine the applied strains in film substrate composites using both X-ray diffraction and digital image correlation measurements. Such an experimental combination is used for the first time to determine the yield surface of a polycrystalline thin film deposited on a polyimide substrate. In situ biaxial tensile tests under different biaxial planar load ratios were performed on W/Cu nanocomposite thin films deposited on flexible substrates. The effect of loading path on the yield stress of W/Cu nanocomposites is presented by considering a large range of proportional loadings. By comparing experimental results with theoretical models, this study reveals the brittle behaviour of W/Cu nanocomposite thin films at small deformations.