I don't know about that one, but have seen the double engine 750's before.
A guy around where I used to live built them as customs for people in the mid 70's. They looked stock except they had an extra engine, he did a beautiful job on them. I think they called him Fast Eddie and he loved to ride his dbl engine through Laidlaw Harley (he knew the people there) and tell them that the 16" Harley wheel was so at least one Harley part would know what it was like to go fast.