Bilayer hydrogels encoded with smart functions have emerged as promising soft materials for engineered biological tissues and human-machine interfaces, due to the versatility and flexibility in designing their mechanical and chemical properties. However, conventional fabrication strategies often require multiple complicated steps to create an anisotropic bilayer structure with poor interfaces, which significantly limit the scope of bilayer hydrogel applications. Here, we reported a general, one-pot, macrophase separation strategy to fabricate a family of bilayer hydrogels made of vinyl and styryl monomers with a seamless interface and a controllable layer separation efficiency (20-99%). The working principle of a macrophase separation strategy allows for the decoupling of the two gelation processes to form distinct vinyl- and styryl-enriched layers by manipulating competitive polymerization reactions between vinyl and styryl monomers. This work presents a straightforward approach and a diverse range of radical monomers, which can be utilized to create next-generation bilayer hydrogels, beyond a few available today.