Waste of effort. You'd make more power with two big singles alone and wouldn't be putting the crank any extra stress.
I've never got why anyone thinks "twincharging" is a good idea.
I sense some jealousy, and for that I award you two Mr. Schnauzies:
http://www.tripledoubleyou.com/wp-content/uploads/2007/11/toaster_trailer_dog_house.jpg
http://www.tripledoubleyou.com/wp-content/uploads/2007/11/toaster_trailer_dog_house.jpg