http://en.wikipedia.org/wiki/Henmaru_Machino
A lil information for a Manga artist, who has done futanari stories before, i've seen the images in a manga artist book myself, but i can't scan them as i don't have a scanner. It appears he done a lot of futanari art within his Yumiko chan stories. One of the Yumiko Chan TG stores can be read at Farhad's site, but it's more mind swap then process.