In my personal experience I have five vivs, three with pairs of cornsnakes in and two with a single cornsnake. My pairs of corns live together without stress and seem happy, one of my single corns is a very techy snake and doesn't like any other snake in with him, and my last single snake is a baby corn and soon will be moving into a newly built viv with another baby corn and will share all being well.
I find that two males do not live together happily, but a male and female seem fine.
I do agree with the parasite and disease thing, if you have only one snake per viv you do not risk infection to other snakes.
I also agree about feeding the cornsnakes in seperate places because one of my snakes tried eating the others mouse, and ended up with her head in his mouth, and the only way to seperate them in this instance was under a cold water tap ( which I will never do again because I realised afterwards the shock could have killed both of them).