The whole world is looking for imperative technologies that can produce clean water, clean energy and clean environments with low energy consumption and small footprint. Membrane technology stands out as a potential solution for different applications because of its beneficial advantages. The revolutionary dual-layer (DL) membranes are composed of two layers formed by co-extruding or co-casting two different dope solutions simultaneously in one step. With the advances in dual-layer concept, one can effectively design the membranes with the maximal performance from various polymeric pairs. As a result, DL membranes are increasingly being studied for gas-phase and liquid-phase separations. However, challenges still exist such as parameters or conditions affecting the adhesion/delamination in DL membranes, the methods to fabricate delamination-free membranes, the knowledge to bridge the fabrication conditions and their influence on membrane performance. Therefore, this review not only summarizes the state-of-the-art research on DL membranes but also provides critical insights on these scientific challenges and potential solutions. A future perspective on this technology including other challenges is also given.