It's a kind of a phylosophical question, not mathematical.
According to the bible, when a man and a woman marry and become united, they become one flesh together- so a man which is 1, and a woman which is also 1 become together.. guess again... that's right.. 1...
Think the answer may have been starring us in the face all the time but no-one could see it. Im not perfect at mathematics, I prefer physics and that sort of stuff, the general and specific theories of relativity are mind riviting things to read.
Anyway lets get back on topic lol. I think the answer lies in Biology and reproduction and a lovely little thing called Deoxyribonucleic acid, shortened to DNA. When one set of female DNA is joined by one set of male DNA, the result is one complete new set of DNA, either male or female.
Therefore 1+1 = 1. Quod Erat Demonstrandum.
Mind you another possible answer is that the numbers are Binary numbers where 1 plus 1 equals 1. Thats what MummyMia reckons.