Please ask vendor to provide Axial, Transverse and Torsional spring rates for each bellows along with detailed dimensions for the tied-universal expansion joint and accordingly, modify the default dimensions and spring rates from Caesar-II EJ modeler; so as to get similar overall lateral spring rate. To test this, prepare separate Caesar-II file for this EJ alone, keep one end anchored and apply some nominal lateral force say 1000N at the other end of EJ and check corresponding lateral deflections at free end in the output report. Overall Lateral stiffness = Lateral force / Lateral displacement.
There are different modelling methods for universal EJs, simple vs complex explained in CAESAR II Applications Guide.