The subject of this paper is a two-carrier planetary gear train (PGT) which was developed for a specific purpose. This PGT may be used for applications in which a negative transmission ratio in the range -3…-143 is required. In this case, the mechanical and dimensional properties of the PGT have been analyzed for nominal negative transmission ratios of -30 and -40. All possible combinations of simple component PGT ideal torque ratios providing those transmission ratios were obtained. Only the combinations providing the minimum radial dimensions of the PGT assembly were selected. Subsequent analysis has shown that the PGT radial dimensions will be minimized when the ratio of the reference diameters of the planetary unit ring gears is close to unity, meaning that the PGT casing will be cylindrical rather than stepped. This paper also provides an overview of the DVOBRZ software package used to synthesize the different gearbox layouts for the required transmission ratio, combined with a basic introduction to single speed two-carrier PGTs for better understanding. The DVOBRZ software was used to select all the acceptable gearboxes were selected from the set of generated PGTs according to the criteria of minimum dimensions and acceptable efficiency range. This set of PGTs was checked for kinematic feasibility and construction concepts were created for feasible layouts.