so are you suggesting they are shapeshifters or live in duality? or is it comparing apples and oranges?
Only if the men and the Tsaesci are apples and the mer and Ka Po' Tun are oranges.
It's metaphysics - kinda. You see, Ka Po' Tun are (play the same role as) elves because they share the same "mindset". Elves, or more specifically Altmer, is trying to breed themselves back into Aldmer, and their "god selves". As you may or may not know, Altmer belives they descended from the gods, and is actively seeking to reshape themselves back into their ancestors. Ideas on how this could be done has differed greatly - The Dwemer tried it, Mancar Camoran had his own thoughts on it, and then there's the Altmer mainstream theory, of course.
Now, how does this relate to the tiger-people? From http://www.imperial-library.info/mwbooks/mysterious_akavir.shtml:
A great war was raged, which left both the cats and the snakes weak, and the Dragons all dead. Since that time the cat-folk have tried to become the Dragons. Tosh Raka is the first to succeed. He is the largest Dragon in the world, orange and black, and he has very many new ideas.
Firstly, "dragons" is a common paraphrase for gods.
Secondly, the "great war" is referring to the war between Lorkhan and the Aedra - a war in which the "elves" sided with their gods and the "humans" with Lorkhan. This was fought all over the world. When it was over, many "gods" (Ada or Aedra) had died, and become "Earthbones" (and, according to Altmer myth, Aldmeri elves as well).
Since then, the Ka Po' Tun have tried to become dragons - gods - again, just like the altmer. They share the same role in the "metaphysical play", just featured in two different productions.
That is also why the Tsaesci are so commonly referred to as "men", or as having been men, or as having eaten men - they also share the same roles. Even so, they are Immortal Vampire Snakemen, and not anymore human than the cat-people are elven. They're the same, but different, the same way Hamlet wouldn't be depicted exactly the same in two different theatres.