I had a set custom-made a couple years back, so I could run matching 8-lug wheels on both my front D60 and my 6-lug Toyota rear axle. The bolt patterns overlap in a way that will eliminate two opposing the studs on the 8-lug pattern, so I ended up with all 6 lugs holding the spacer to the axle, then 6 of the 8 holding the wheel to the spacer.
I think other companies (MC motorsports for one) will make two-piece adapters so you can use all the studs--but they're necessarily thicker and more expensive because of the complexity.