nightfall second profession
malachore
when can i change my second profession?
Kook~NBK~
When you get to the Chantry of Secrets, about 1/2 way through.
Faust Bane
Once you have completed the mission Building the Base: Prisoners of War, Zuwarah will appear in the Command Outpost and she changes your secondary.
Kamatsu
As Faust Bane said, once you have gotten the Sunspear Sancturary and have done the "building the Base" quest's you'll have access to a NPC who can change your secondary for you.