It's been stated time and time again that Isam/Luc can choose to step between one or the other personality, at will, but only when entering or leaving TAR. It is a conscious choice he makes at the moment he moves from one to the other and he can choose either personality at any transfer.

Here, this is from an entry by Linda at The 13th Depository in her article on Isam Mandragoran:

- Another of these gifts is the ability to shape-shift. On exiting or entering Tel’aran’rhiod, Slayer can choose whether to ‘be’ Luc or Isam:

Slayer just chooses who he will be when he steps into or out of Tel'aran'rhiod. The stepping in and out is part of the mechanism for his change. He couldn't do it in the middle of a street, say, not without the stepping in or out. Which might be a little noticeable, since he would vanish from sight for a perceptible time.

Robert Jordan on his blog

This includes their persona, appearance and scent. Both Isam and Luc have hardly human scents, probably because they enter Tel’aran’rhiod frequently in the flesh, which results in a loss of the person’s humanity. Slayer is a shape-shifter who changes human form, while Perrin shape-shifts mentally from man to wolf. Perrin worries that he might lose his humanity when he does this. Isam/Luc presumably do not worry about losing their humanity.

I think that clears up any misunderstanding on the matter, yes?

(And please forgive me, I'm terrible with the html codes to make all that look the way it should...)