Between Luc/Isam and Moridin/SH. There is no concrete evidence that Moridin and SH are basically one creature and can change their form.

Like it is implied in the books and directly told by the authors, SH is basically an incarnation of the DO in the real world, while Moridin is the most trusted Forsaken. This explains why Moridin and SH are closely working together. Nothing suggests that RJ was trying to fool us about SH's or Moridin's nature.

For example the following report doesn't make much sense, if this theory were right:
"Brandon hinted at some severe limitations on Shaidar Haran to affect the physical world. He says that a lot of actions that people assume to be those of Shaidar Haran in the book in one particular scene were physically carried out by Elza. He further indicated that Shadar Haran would have been incapable of physically placing the collar himself."

SH could simply switch to his Moridin persona to circumvent such limitations to affect the physical world, since Moridin obviously can affect the physical world with no problems. But SH can't do that, because Moridin is someone else.