Embodiments of three-dimensional (3D) memory devices having multiple memory stacks and methods for forming the 3D memory devices are disclosed. In an example, a 3D memory device includes a first device chip, a second device chip, and a bonding interface. The first device chip includes a peripheral device and a first interconnect layer. The second device chip includes a substrate, two memory stacks disposed on opposite sides of the substrate, two memory strings each extending vertically through one of the two memory stacks, and a second interconnect layer. The bonding interface is formed vertically between the first interconnect layer of the first device chip and the second interconnect layer of the second device chip.