There is a local pet store here where I live that has a pair of these(black rat x normal corn). They are cohabbing!!!
They co-hab because it's only temporary, until they sell the snakes, also, they sell the snakes before sexually maturity so there is no chance of the female become egg bound (if they are a decent pet store at all!!)
Tania is right, if you co-hab them you put them at all sort of risks, the most likely being the female breeding too young and dying. The worst being they eat each other. Snakes are not social creatures, they probably are stressed.
Also, the two black rat's I have met have been sweet hearts. While some behavior can generalized based on species, it really comes down to the individual snake. If you want a nice pet, just make sure you find a nice black rat before you buy one. I wouldn't get the "mean" one from the pet store (well, I might because I like mean snakes, but I'm guessing you don't, lol)
My advice is to wait, there are so many more snakes out there than what you see in the pet store and fall in love with on any give day. Hang around here, look into a lot of different species, pick one that suits you and start preparing to purchase one. Why just stick with colubrids? Spotted pythons are nice and so are rosy boas. Variation in a collection keeps things from getting boring.
And don't be afraid to sit back and read/edit your post before hitting that "submit" button, fixing the grammar up will go a long way in getting people to take you more seriously.