Sometimes, when you want to ride on two beasts you will have to do the trick. It can be pleasure if they hit the right spot however, be cautious, forcing your body without good consideration and your butt will be out of order for a time along with bleeding and unwanted pain of course, not to mention walking disability and... you will have to lie with your face down until the condition improved )

This usually happen in yaoi when you are raped.

http://www.mangaupdates.com/series.html?category=double%2Bpenetration& genre=Yaoi&perpage=50
All of these have double penetration