I'm not sure about hydras. Some depictions hiss and snap between heads, some don't (by design or omission, I don't know).
This might be kind of relevant. I was reading something about the mental capabilies of animals, and according to a study snakes lack a centralized awareness:
It seems that a snake does not have a central representation of a mouse but relies solely on transduced information. The snake exploits three different sensory systems in relation to prey, like a mouse. To strike the mouse, the snake uses its visual system (or thermal sensors). When struck, the mouse normally does not die immediately, but runs away for some distance. To locate the mouse, once the prey has been struck, the snake uses its sense of smell. The search behavior is exclusively wired to this modality. Even if the mouse happens to die right in front of the eyes of the snake, it will still follow the smell trace of the mouse in order to find it. This unimodality is particularly evident in snakes like boas and pythons, where the prey often is held fast in the coils of the snake's body, when it e.g. hangs from a branch. Despite the fact that the snake must have ample proprioceptory information about the location of the prey it holds, it searches stochastically for it, all around, only with the help of the olfactory sense organs (Sjolander, 1993, p. 3).
Finally, after the mouse has been located, the snake must find its head in order to swallow it. This could obviously be done with the aid of smell or sight, but in snakes this process uses only tactile information. Thus the snake uses three separate modalities to catch and eat a mouse (Dennett, 1995b, p. 691).
So in this case, a hydra would probably be more mentally advanced (because of magic? or just because of how complex they are) than a snake but it seems there's somewhat of a precedent of 'separated awareness' in some reptiles. They all work together in a snake, so maybe it's just a bit more complex in hydras (several sets of sensory awareness instead of one).
As for Ettins, I doubt this would come up in gameplay in any way at all, but split brain studies
http://en.wikipedia.org/wiki/Split-brain (where, for medical reasons, the bit connecting the two halves of the brain is split) causes some interesting things to happen.
Of course, this is probably too biological and not magic enough for a fantasy game, I just wanted to share my thoughts.