Oaky, here's what i want you to do:

1. make a cube.

2. duplicate this cube.

3. hide the first cube so you're only working with the duplicate.

4. select the TOP of the cube and drag is down past the bottom of the cube.

5. find something along the lines of "reverse vertex order" that will cause all the selected vertexes to flip over, and use this on the second cube.

6. unhide both cubes and delete the faces that are now inside the larger cube you've created from the 2 smaller ones (ie delete anything that can't be seen from the outside)

7. proceed with the rest of the instructions.