To load multiple models which are defined in different unit to the scene. For example, a model is defined in millimeters and another model is defined in inches.
The model will be scaled to the unit value of the current scene with this setting Model.setEnableAutomaticUnitScaling() as explained in 
Model Units — HOOPS Communicator documentation
Alternatively, if you want to combine two models before converting SC, you can do it using HOOPS Exchange API ( if you are using HOOPS Exchange.)
You can import two files and combine the models to a prc file, then convert to SC using Converter.
You can find the sample code for HOOP Exchange from this article. How-to: Combine multiple models with different unit