Todd, it seems a bit confussing at first...but I will try to detail the steps (and trust me I am as novice where this is concerned as possible).
1. Compose the image on the GG of the 8x10 (a line for the 4x10 will help frame the subject).
2. Insert your normal 8x10 holder with the 8x10 darkslide in place.
3. DO NOT open the shutter yet, now withdraw the 8x10 darkslide
4. Insert the cut down 4x10 darkslide, with the open area matching the half of the GG you framed the image in.
5. Now expose
6. Insert the 8x10 darkslide, making either a mental note or using some other method (tape, etc) to mark the side of the film that was exposed.
7. Remove the film holder, and recompose the shot - using the OTHER half of the 8x10 GG (so if the first shot was done using the top 4x10 half, now compose on the bottom 8x10 half).
8. Insert the 8x10 holder, with the same side you exposed previously (let's say it was the top).
9. Do not open the shutter yet
10. Remove the 8x10 darkslide
11. Insert the 4x10 darkslide (the same one, but this time insert it so that the top of the film is covered).
12. Expose the bottom half of the film
13. Insert the 8x10 darkslide and remove the film holder
14. You will now have two (2) 4x10 images on one (1) 8x10 sheet of film
Process and pat yourself on the back for a job well done -
Actually the process would be the same using a slider board as well, you just get to see the half you expose. Slider on Top = expose the bottom half of the 8x10 sheet of film, slider on Bottom = expose the top half of the 8x10 sheet of film. It does require a bit more concentration to keep up with what you are doing, but the results are well worth the extra effort.
Good luck and let us know how it works out.