:Can they? Yes. Should they? That's debatable. I house mine together as they are all CB and free of parasites and disease. However, I wouldn't put a wild-caught snake in the same tank as CB animals. The potential to spread parasites and disease is just too great.
:Other than that, watersnakes are generally very communual animals and often can be found basking in close proximity to, and even on top of, one another in the wild. There is little to no risk of them eating one another though you should always feed them seperately!! I have had baby watersnakes fight over fish and they are so tenacious that they will not let go. Eventually one might end up swallowing the other.