The nonlinear development of double tearing modes (DTMs) mediated by parallel electron viscosity in a large aspect ratio torus is simulated. The emphasis is placed on the mechanisms for the fast growth and sheared flow generation in the development of the DTMs. Four nonlinear developing stages: the early growth, transition, fast growth and decay, are found. In comparison with the helical magnetic flux contour, the fast growth is revealed to be associated with the reconnection and annihilation of the magnetic islands formed by the reconnection of equilibrium magnetic field in early stage of the mode development. The quantitative scaling of the growth rate and the flow shear with respect to electron viscosity is presented. The stabilization effect of a vortex flow generated in tearing mode development on ion temperature gradient modes is also demonstrated stronger than that of a mean or streamer flow of same magnitude.