We know from RJ that there was a previous version of SH in TDR, when Ishy was still around, SH 0.5. As said, we also have a SH PoV, where he spies on Sammael & Graendal, muses on his limitations and states that he is a Myrddraal (though a special one) and can't channel the OP (but use the TP). RJ later elaborated on this, telling that the DO projected his shadowy self into the body of a Myrddraal. SH is "the Dark One in shadowy form" and this all the time ("twenty-four/seven" as RJ put it). On the other hand, Moridin thinks of the DO as a separate being.