I'd say Rasu has it well enough. In the most absolutely simple terms I can manage, you need two rivals, a goal, and a witness. The two rivals become so wholly defined by their goal that they become indistinguishable to the witness, and since belief is reality in TES, that means they are indistinguishable, at least at that moment.
Because Anu and Padomay existed before there was time, they are indistinguishable but still distinct because there was no linearity to enforce it on anyone who came after. Since the next set of them was Auriel and Shor, one being time and the other space, they were also not bound by linear time (or became Space-Time as we understand it, I'm a little fuzzy on that actually). Since every enantinomorph after becomes one or the other of them, they stay themselves while becoming the other.
So if we go back to Tiber -
The Almsivi were the witness and Tiber, Underking, Arctus etc. etc. were their rivals, their goal was to unite the whole of Tamriel and got so wholly defined by it they became indistinguishable to the Almsivi, becoming "one"?
Is that the sort of thing?
In "Nu-Hatta of the Sphinxmoth Inquiry Tree" Tiber also said the Stormcrown (himself) mantled, I take it into Lorkhan, therefore causing all the other enantiomorphs to become Lorkhan.
Now I found something that perfectly explained Mantling for me:
"Another byproduct of mythopoeia is the process of mantling, or acting like a deity in existence so that those perceiving the actor cannot tell the difference between the ‘mantler’ and the ‘mantlee.’ Such was the process by which Tiber Septim achieved divinity, becoming the Cyrodiilic god we know as Talos. Talos and the Underking acted in a way that mythically echoed the struggle between Lorkhan/Shezzar and Auri-El/Akatosh. Thus, did Tiber claim the ninth place in the Cyrodiilic pantheon where Shezzar had not been."
So now I get that enantiomorph and mantling part, since one acted as Shezzar and one acted as Akatosh, both echoing that stragle but since both were the same through enantiomorph-tion, they became both.
Am I getting this all right?