Recent microscopy experiments have investigated the structure of mixed adsorbed protein films. These studies have provided conflicting messages about the extent of miscibility of different proteins at fluid interfaces. We address this problem here using a Brownian dynamics simulation approach. We find that formation of bonds between model protein molecules can lead to either phase-separated or homogeneous mixed film structures, depending on the attributes of the bonds. We compare the behavior with that for a simulated case where an effective repulsion between unlike species can lead to phase separation, and we discuss the overall relevance of the model to protein mixtures.