How to assemble few parts that will be made of different materials?

I want to create a thin sheet made out of parts with different stiffness by using different materials and their combinations. That, by creating a smaller sheet that I made in the proper dimensions and converted to STL file, duplicating it in GrabCAD and then picking a certain material for each piece. The parts should be connected to each other as a continuous part, without support material between the parts.
When I tried duplicating the part, it just collided with the first one.