In the search of multiferroic materials with enhanced magnetoelectric response, the BiFeO3-Bi4Ti3O12 composite system has been proposed as a promising candidate. However, in order to ensure an effective coupling between the antiferromagnetic and the ferroelectric-order parameters, a high structural quality of the oxide heterostructure (well-matched interface) must be attained. This implies the absence of any inter-diffusion process across the interface during the consolidation of the composite assembly. In this contribution, we have analysed the different diffusion scenarios that could be established in this nominal BiFeO3-Bi4Ti3O12 system as a function of the specific reactivity of the involved compounds. The obtained results clearly identify how and when that diffusion is produced, so now it can be controlled to ensure the maximum exploitation of the potential multiferroic properties of this candidate system.