Half the Solution

You get this guy and his twin together, and then could they simultaneously fuck one another?

Something that I hadn't considered before is that it the "Single Double" were to be performed, there would be four balls being pushed around in a small space between the cocks and the assholes.

Unless I hear from someone who has performed it or knows of someone who has performed it, or has seen a video of this, I'll just chalk it up to dreams on my part.