Mixtures of two diblock copolymers of very different lengths may feature both macro- and microphase separation; however, not much is known about the mechanisms of separation in diblock copolymer thin films. In the present work, we study thin films of mixtures of two compositionally symmetric block copolymers, both in the one-phase and in the two-phase state, combining coarse-grained molecular simulations (dissipative particle dynamics, DPD) with scattering experiments (grazing-incidence small-angle X-ray scattering, GISAXS). We reveal that the film thickness and selective adsorption of different blocks to the substrate control the distribution of macrophases within the film as well as the orientation of the lamellae therein. In thick films, the mixtures separate in the vertical direction into three layers: Two layers being rich in short copolymers are formed near the film interfaces, whereas a layer being rich in long copolymers is located in the film core. The lamellar orientation in the layers rich in short copolymers is dictated by the surface selectivity, and this orientation only weakly affects the vertical orientation of lamellae in the film core. This provides the opportunity to control the domain orientation in the copolymer films by mixing block copolymers with low-molecular additives instead of relying on a more complicated chemical modification of the substrate. In thinner films, a lateral phase separation appears.