The industry adoption of memristor-based technologies hinges upon improvements in several key performance metrics for these devices, including controllable switching, multiple resistive states, reducing the variability over multiple switching cycles, and reduction in the switching and forming voltages, depending upon the specific requirements for different application domains. Of these, a new kind of memristor comprising two different metal oxides as active layers between inert electrodes, referred to as the bilayer resistive RAM (ReRAM), has been shown to possess gradual analogue switching characteristics [1], making it particularly suitable to act as the variable coupling element between oscillators within the energy-efficient neuromorphic computing framework of oscillatory neural networks (ONNs) [2,3]. However, the atomistic mechanisms underlying the unique behaviour of bilayer ReRAMs remain to be uncovered, which is crucial for understanding and further optimization of this technology for its integration with the ONN computing hardware. Another peculiar characteristic of this device is the high voltage requirement for the forming step, which is the first process when the initial structural changes related to the resistance-switching behavior occur in the pristine device [1]. In this work, we reveal the detailed ionic response in this device to understand its filamentary forming mechanisms and explain its observed characteristics using atomistic simulations. For this, we use an implementation of a novel molecular dynamics simulation framework, integrating the local electrochemical potential formulation (called EChemDID) [4] with the charge transfer ionic potential (CTIP) [5,6], to capture the accurate physics of this device. We clarified the differences in the peculiar responses of each of the anionic and cationic species in the pristine structure in response to applied voltage and joule heating in the device, and the physical reasons underlying these mechanisms. Our results indicate that an oxygen-rich region could form on top of the filament in bilayer ReRAM devices, as a consequence of repulsion of the Tantalum ions that occurs even at lower temperatures. Furthermore, the filament initiates at the cathodic interface of the active MO layers by agglomeration of the vacancies when the applied voltage is beyond a critical threshold voltage, and an increase in temperature significantly accelerates the growth of the filament, particularly owing to an increased thermally excited mobility of tantalum and oxygen ions promoting bond breakage and vacancy formation. These mechanisms reveal the key processes underlying forming in the bilayer ReRAM, which could be controlled for directed optimization and further improvement of the key performance metrics of these novel devices. This project has received funding from the EU’s Horizon program under Projects No. 101092096, PHASTRAC.