Honestly I don't see any real reason to house two snakes together. I would bet that most of the time it isn't a major issue. But I have always felt that by keeping these animals in captivity it is our responsibility to provide the absolute best living situation for them. So if something even has a possibility of going south I don't want to give them the opportunity. I have seen snakes try and be dominant over another, compete for hiding spots, temperatures, etc. I've seen snakes get stressed out because of cage mates. I've seen feeding accidents. And now I've seen a video of them fighting. But what is the benefit of housing them together? You save some money on housing and heating? If someone has a better reason I really would love to hear it (not being sarcastic :) it's a forum and therefore a place to learn). To me it just isn't worth the risk. Just my opinion.
EDIT: there are some keepers that I have heard say it's acceptable for certain species, but this doesn't include ball pythons