A person with two anus's. More commonly this is caused by an untreated anal cyst which breaks the skin and creates a second passage. There is another condition called genital duplication which is known to produce a second anus in sufferers.
A friend we once had apparently suffered from an anal cyst many years ago which created him a second anal passage.
Because we had no compassion for this person at all, we decided to call him twopoo - and so it was spoken, that from that day forward that anyone with the misfortune of having two arse holes would be known as a twopoo.