How to superimpose one model onto another

There are two ways to superimpose one model onto another. The first way is to select multiple files by holding Ctrl key in the File Opening dialog.

The following picture shows the two selected files at the same time

Another way is to open one model first, then use "Import Model" menu button (under File menu) to add additional parts to the scene. The following picture shows the first model

Then select "Import Model" menu button

Then select the additional files you want to add to the 3D scene

Here is the result