The paper presents the effect of electron treatment on the various properties of flax fibers (20 wt%; 5 mm in length) reinforced PLA/PCL (70:30) composites. To enhance the intensity of the electron-induced cross-linking reactions, triallyl isocyanurate (TAIC) as a cross-linking agent was also added (up to 3 wt%). The composites were manufactured by conventional extrusion, granulation, and injection molding techniques, and then electron irradiated at doses of up to 60 kGy. The structural changes were determined using mechanical testing (tensile strength, impact strength, dynamic mechanical analysis), gel content analysis, thermogravimetry, and differential scanning calorimetry (DSC). The modified composites were also subjected to enzymatic biodegradation and industrial composting. An important novelty aspect of the presented research, additionally to the new material composition, is the use of the rule of mixtures in the assessment of the effect of electron treatment on the interfacial interactions of dispersed PCL in the continuous phase of PLA, in the presence of flax fibers. PCL was dispersed in the PLA phase with an average phase size of a few micrometers. Based on the rule of mixtures, it was found that without TAIC, the predominantly degraded PLA phase-initiated cross-linking of the dispersed PCL phase, contributing to the interphase adhesion enhancement reflected in improved mechanical properties. To apply this model, it was required to refer to the previously published results of analogous composites with homopolymer matrix (PLA or PCL).