We know little of conflicts between Daedra, but Jyggalag mentioned that his dominion "expanded across the seas of Oblivion with each passing era", and Haskill seems to have implied that Daedra do wage war against each other.
Actually taking another Prince's realm is another story. Even if it's possible to sever a Daedric Prince from his realm you would not only have to worry about the defending Prince having the home field advantage (As in the very least they have significant power over their realms) but the possibility of another Daedric Prince invading your own realm while you're away.