Cohabitation is not advised for cornsnakes. Depending on the age category there are different reasons for not cohabitating. In the case of yearlings, if you have a male and a female, there's a good chance the male could breed her before she's ready (yes, even if you don't brumate), and the resulting clutch could very likely kill her, and if not it will atleast stunt her growth. If it's two males, they may become aggressive and territorial with eachother. Two females (if you're CERTAIN they're two females) are probably the least likely to have problems...
Let's put it this way... Don't Cohabitate is one of those golden rules that everyone should follow. You might be able to bend the rules and get by (there are people out there that cohabitate and don't have issues), but the list of different issues that can occur with cohabitation is extensive enough that even those with enough experience to know when those golden rules can be bent or broken will refuse to break that rule, especially with anything beyond brand-new-hatchling stage.
Think of it like stepping out in front of a moving car. Yeah, the driver will probably stop if you're careful about it. Yeah, your friend got the driver of a different car to stop. But do you really want to take the risk the driver isn't paying attention?
-Kat