I did something. I don't remember what, but I was able to directly (not through an intermediate STL file/files) open all sixteen bodies in Simplify3D, so that they all showed up under "Models" with individual checkmark boxes. I was then able to group the text, and use the dual-extrusion wizard to select the sign as the "white" extruder and the text as the "red" extruder. It worked so well, and I'd spent hours trying to get to that point.
I celebrated rather than taking good notes, and now I'm extremely frustrated because I can't reproduce this workflow.
I tried a very simple test: create a 50mm white cube with a 25mm red cylinder 25mm deep in the center of the front face. I can see where I have two separate bodies, and I can even make one white plastic and the other red plastic. But I can't, for the life of me, get these to open up as a group of two objects in Simplify3D.
Help?