Yes, this would be similar to a boolean OR operation to merge the one file down into the second file. In 2D, this is called "welding" by Aspire.
I can't remove material from either model, as both extend essentially all the way to the bottom of their respective sides. This is a guitar neck, as I said before. One side has the curved back of the neck on the "top" and the other has the headstock shape and truss rods on the "top". The models don't overlap in the real world, so there are no cut throughs, except along the edge, as desired, and I can carve them, with some challenge, as separate files. While I don't really care which one I start with, I want to merge them together to form one two-sided file of the correct thickness. If I remove 10mm from either side, or if I reduce their heights, I will distort or damage the features I need, which need to be accurate to within a mm.
"The 5 inch model would be inside the 7 inch model if the final model was to be 20mm thick" - this is exactly what I want to do (only in my case, both front and back would be the, let's say, 7 inch disk.)
Aspire can merge two shapes in 2 dimensions on either side by welding (which is what I'm more or less trying to accomplish), can it do the same thing in 3 dimension front and back?
I've attached a picture, shown in 3D, from an oblique angle, showing the part twice as thick as required, with a plane separating front and back. I want the plane of the front side lined up with the face of the back side, and the plane from the back side to line up with the face of the back side.