I'm not sure if annibalism has been observed in Hydrodynastes or not (though I tend to believe it has been), but they do have insatiable appetites, so housing two specimens together (especially where one is significantly larger than the other one) is not advisable. Females tend to be larger and grow faster than males, so the chances that you would have two similarly sized specimens (for very long at least) are not great. In addition, they tend to be VERY excitable during feeding time, and dealing with two food-crazed snakes at one time is not fun at all. Feeding them seperately ensures having to only deal with one insane snake at a time.
Personally...I really don't like housing any snakes together unless it's for breeding purposes. You can do it for many different species, but in my opinion, Hydrodynastes is not one of them.
As for breeding...unrelated specimens are ideal, but breeding related individuals together is not neccesarily a bad thing...the entire morph market is based on inbreeding (at least in it's early stages). As long as snakes aren't inbred over generations, the likelihood of negative traits making themselves known are pretty slim.