The numerical simulation of interaction between structures and two-phase flows is a major concern for many industrial applications. Using a discrete forcing method (see the work of Benguigui et al) (implemented in a multiphase CFD code based on a two-fluid approach) to track the solid motion in two-phase flow, an iterative fluid-structure coupling is developed to allow free-motion of multiple solids (with any kind of geometry) due to two-phase fluid forces. As the fluid-structure interface is located thanks to a time and space dependent porosity on a cartesian grid, the fluid force computation is accommodated to the interface tracking method. A Newmark algorithm is used to estimate the solid motion. The iterative coupling is addressed in detail going from the algorithm to the determination of its convergence parameter. Three application cases are proposed to validate the method from motion under a single-to a two-phase flow.