I really think any snake keeper draws its own lines what is acceptable and what is not. We should respect that, as long as a keeper carefully watches his animals and never let them in a situation they are apparently don't thrive.
I take my husbandry seriously although I am a n00bie. I do not think anyone with less than many years of experience should cohab. But I know that Kathy Love discusses having done it in her book, and I think in the hands of someone with her level of expertise, it could be done safely because a person like that could detect which snakes tolerate it & thrive and which do not.
I also wish that we as a group would be willing to provide support to a study done by someone like her that would give the rest of us good guidelines.
I have read that garters can cohab safely in large vivs. That might be nice for someone who would like to cohab a colubrid species if it's true.
I have also read that some boa breeder cohab for the mating season, separating to feed but allowing them to live to together and mate when they wish. Those breeders could provide info on whether, say, 2 females would do well together if provided with more than enough space & separate feeding areas.
For the rest of us who lack these levels of expertise, I think we should stick to housing them singly! If you are worried about lack of space, the 66Q racks Nanci has look pretty spacious. Or a rack designed for smaller boids would be quite spacious for an adult corn while still providing compactness for the owner.
Of course, this is IMHO and YMMV. I have noticed European keepers may do more cohabbing, but from the pictures I have seen that they are cohabbing in very large vivaria, and I have gotten the impression that the Europeans who cohab colubrids are highly experienced with their species. So if you fall into those categories, MHO may not really be applicable to you, and I wouldn't criticize, and Blutengel, you sound like you are one of those folks, so don't take any of this as aimed at you. It's really aimed at folks who use tubs or racks and don't have a high level of experience