Fabrication of electronic and optical devices involves synthesis of tailor-made materials with complex composition. Reactive co-sputtering from two targets is a suitable process for fabrication of, e.g., a two-element nitride compound material. The composition of the two-element compound, however, is strongly dependent on different processing parameters. It is therefore of great importance to understand and study how the processing parameters influence the composition of the deposited film. In this article we present studies of the film, composition in reactive co-sputtering of titanium and vanadium nitride. In order to determine the film composition during film growth we have applied in situ soft x-ray emission and optical emission spectroscopy. The experimental findings are compared with ex situ elemental analysis and predictions from computer simulations of the reactive sputtering process. The experimental results agree qualitatively with the results from the simulations. Our findings also illustrate the complexity of the reactive co-sputtering process.