Dual-bell nozzles represent a possible solution to improve the performance of large liquid rocket engines for launcher first stages. The present paper studies the role of the second bell shape on the side loads that can occur during the transition between the two main operating modes of the nozzle. In particular, the design of the second bell profile is critically discussed on the basis of results obtained from suitable test cases. The analysis of performance and behavior during the transition is carried out by a validated turbulent Navier-Stokes solver. Geometries were generated by the method of characteristics. Results show that slightly different geometries designed by the method of characteristics yield modest performance changes but severe differences in behavior during the transition phase.