Assuming that an omnipotent being could exist, can more than one coexist each other?
Suppose there are two omnipotent beings -- X and Y. If there are more than one omnipotent being, that means neither of those omnipotent beings is less in its power. That is, neither X < Y nor X > Y. We can't say X ≮ Y or X ≯ Y either. That means X = Y. But this also is impossible because X or Y can't delete or destroy the other. That means neither X nor Y is omnipotent. So a uniqueness is essential in the case of an omnipotent being.
It could if you'd restrict the meaning of omnipotence as was done in Fantastic 4 by introducing creatures, who could perform 4th wall breaks and turn to everything, thus not being "normal" The question whether one omnipotent being could kill another, is more of history than possibility. How would a mortal being know of it, if its creation is in a world, what happened after that event, and the "god" who created this world didn't choose to share it with his world, because he did it; deemed it irrelevant or for some other reasons? It would transcend our understanding of life/death and existence. For instance, could one omnipotent being augment or diminish another omnipotent being - make him become a mere mortal like the fall of Lucifer?
The motive of this question is an inference like, "If a being is omnipotent, it can do anything; having different arithmetic would be something; so an omnipotent being can have a different arithmetic." But if 1 + 1 = 3 is a logical (enough) truth, then...
In other words, if an omnipotent being can do "anything," then it could be omnipotent without being able to do everything---in fact could be omnipotent even if it couldn't do anything at all, since there is no "logic" to worry about. But really, talk of this sort of omnipotence is empty, then. We need to ask questions about specific ranges and types of powers. It is enough for piety, then, to say that a divine being has unique and maximum powers, but not all powers whatsoever (e.g. a being who has no physical hands cannot grab things with hands, though they might move others to grab things).
It depends on your conception of omnipotent being. If it has the potential to be unique, it will. Does your concept of omnipotent includes such potential?
The traditional problem of omnipotence comes from the concept such potential represents. It's quite common for omnipotent being dreamers to get in conflict when the potential is confronted to logic. Can such power create an object that he can't lift? Can he affirm he's wrong? Can he fail?