Answer is ... both.
Although they have common ownership, Highland and Atlantic have a separate management, separate AOC and separate flight ops / recruitment dept.
Atlantic are the only ones with ATR's at the moment, but Highland are getting some very very shortly.
Couldn't tell you whether Atlantic are recruiting ATR drivers or not, but Highland definitely are, they ran an advert in Flight a couple of weeks ago. Particularly interested in Captains or high time FO's with quick command potential.