Problems from housing two snakes together:
-accidental mating
-one gets sick, they'll most likely both get sick
-one snake will dominate the other (it'll look like cuddling) and the one being dominated will get stressed out which can cause a whole new list of problems
-cannibalism. not all that common but it can happen and it's not pretty when it does
-if one snake regurgitates or has unhealthy looking poo, how do you know which snake it's from?

I'm sure there are other reasons but I can't think of them at the moment. I bought my snakes at Petco where I was told they should be housed together. Once the people on this site clued me in, I separated them immediately and was amazed at how soon I saw a difference. They both started looking and acting healthier. And they grew much faster! Hope this helps!