Multiple "heads," like different colors in a regular printer.

One material was the "working" material, the other was a special material that reinforced the structure while the printing was taking place, and then was dissolved out of the primary material later. This allowed construction of very delicate and complicated structures. I played with a sample crescent wrench. Adjustable. It was molded in one pass, and it functions. Same for a screw-cap bottle, a set of working meshed gears, and other things.

I don't know what would be required to do widely varied material combinations like metal or plastic, but I have confidence in the creativity of those working on these things.