One of them will shift to left field. Both are good outfielders. Then there's the DH. Hicks and Ellsbury man center, I presume. Brett Gardner's playing time diminishes.
I know that Brett Gardner is very popular with Yankee fans but he's really an average player for a starter. Stanton replacing Gardner is a massive upgrade. I'm guessing the Yankees would look to trade Gardener but he'd be great 4th outfielder if they kept him. He's a good defensive player who can play all three positions adequately, is a decent batter, and would make for a very good pinch runner in late inning situations.