The interference phenomenon among multiple flexible cylinders is an important factor in designing offshore structures, which has been extensively studied by researchers in the past decades. In this paper, Vortex-induced Vibration (VIV) responses of two flexible cylinders experiencing the stepped flow with tandem arrangement are mainly studied using the viv-Thick-SJTU solver, which is developed based on the open source toolbox OpenFOAM and the Bernoulli-Euler bending beam model combining with the thick strip model. Hydrodynamic forces are calculated in each three-dimensional (3D) fluid strip, while vibrations of the cylinder are computed using the Finite Element Method (FEM) through the Newmark-β algrithm. The dynamic mesh technique is adopted to update the computational mesh through the cubic spline interpolation algorithm at every time step. Effects of the gap ratio between two tandem cylinders on vibration characteristic are mainly studied, while gap ratios are set as 4 and 10. Firstly, the validation of the code is conducted. Then, the modal decomposition method and Fast Fourier Transform (FFT) method are used to get the dominant vibration mode and vibration features of both cylinders.