We report a system consisting of a mixed Langmuir monolayer, made of water-insoluble, spreadable, fluid-like polymers polydimethylsiloxane (PDMS) and polymethylmethacrylate (PMMA) with a minority P(DMS-b-MMA) copolymer. We have performed both Langmuir trough pressure/area isotherm measurements and Brewster angle microscopy (BAM) observations and complement the experiments with molecularly detailed self-consistent field (SCF) calculations. PDMS undergoes a layering transition that is difficult to detect by BAM. Addition of PMMA gives contrast in BAM, now showing a two-phase system: if this would consist of separate two-dimensional (2D) PMMA and PDMS phases, a PDMS PMMA diblock should accumulate at the phase boundary. However, the diblock copolymer of PDMS-PMMA failed to show the expected "lineactant" behavior, i.e., failed to accumulate at the phase boundary. The calculations point to a nontrivial arrangement of the polymer chains at the interface: in mixtures of the two homopolymers, in a rather wide composition ratio, we find a vertical (with respect to the air/water interfacial plane) configuration, with PMMA sitting preferably at the PDMS/water interface of the thicker PDMS film, during the PDMS layering phase transition. This also explains why the diblock copolymer is not a lineactant. Both PMMA and P(DMS-b-MMA) are depleted from the thin thick PDMS film interface, and the line tension between the phases is, consequently, increased, in the binary mixtures as well as in the ternary ones.