I use SolidWorks and Flashprint. I used SolidWorks to put two parts together to make an assembly. I saved the assembly as STL, using the option to make separate files for each part. Then I loaded each stl file into the printer software. They had the same relationship or mating as they did in the assembly. I specified an extruder for one model (STL file) and the other extruder for the other model. If you need to rotate or move the models, they must be moved together or the same way.