The present invention relates to a twin-frame bicycle, particularly to one that can be pedaled by two riders in side-by-side coordination so that the riders and the twin-frame bicycle will incline integrally without tumbling or overturning. At least one articulation mechanism is disposed between a primary bicycle body and an auxiliary bicycle body, wherein the articulation mechanism serves in synergy an articulation parallelogram, and a resilient tension mechanism having an elastic element is disposed between the primary bicycle body and the auxiliary bicycle body and counteracts the deformation of the articulation mechanism to maintain equilibrium.