The objective of this study was to better understand the gelation behaviour of a mixed soya milk–cow's milk system, by forming different reactive protein particles using rennet and glucono-δ-lactone. The formation of the structure of these mixed gels was followed, for the first time, using diffusing wave spectroscopy and rheology. When only one protein source was induced to gel, protein aggregation was hindered, as shown by the slower increase in apparent radius after the gel point. Confocal microscopy analysis of the gel networks suggested that while milk gels exhibited large pores with interconnecting strands of protein, soya gels appeared as densely packed protein aggregates, and mixed soya milk gels appeared as a network of aggregated proteins. This study demonstrated that by modulating the reactivity of the building blocks, it is possible to fine-tune structure formation of these mixed protein gels.