I was about to mock you for your ignorance of human embryology... then I googled it... holy cow, it is I who needs to go back to fertilization school. I didn't know that 1% of pregnancies result in two sperm fertilizing one egg. that would be a great case... twins with different fathers...
But to the noob who said anal sex can't cause pregnancy... not true, if the semen drips down into the vaginal canal. probably unlikely but if you've gotta a lotta goop, it couldd...