An approach is proposed to expand the range of operational neutronic problems to be solved through combining various models of neutron transport (multigroup with detailed geometry description and two-group with a homogeneous description) in separate parts of the design area. The approach is deployed in the high-precision engineering program MNT-CUDA (version 2.0), capable of full-scale reactor engineering calculations using the Monte Carlo group method with an option of detailed description of neutron transport over the entire system or inside specific areas. The algorithm proposed uses GPU parallel computing. New features of the program are demonstrated. The results of studying the accuracy of combined calculations are presented and analyzed.