One of the possible ways of creating the supermassive black hole (SMBH) is hierarchical merging scenario. Central SMBHs at interacting and coalescing host-galaxies are observed as SMBH candidates at different separations from hundreds of pc to mpc. One of the strongest SMBHs candidates is ULIRG galaxy NGC6240 which was X-ray spatially and spectroscopically resolved by Chandra. Researching of central SMBHs merging in dense stellar environment allows to retrace their evolution from kpc to mpc scales. The main goal of our dynamical modeling was to reach the gravitational wave (GW) emission regime for the multiple BHs model. We present the direct N-body simulations with up to one million particles and relativistic post-Newtonian corrections for the SMBHs particles up to 3.5PN. From our models we found the upper limit of merging time for NGC6240 central SMBHs is less than ~50 Myr.