It's a red. Using both feet is excessive force and that is the red card criteria. It's not even up for debate, as it's nothing to do with getting the ball or making contact with the player. It's the force, and two feet is excessive.
I always remember the Kompany two-footer on Nani where he got a straight red, despite getting the ball and Nani jumping the challenge. I thought that was going to be the moment where referees truly started handling two-footers in a no-nonsense fashion, but it is still a largely inconsistent call. Kompany didn't learn from it, because he still does them all the time.
I don't think they are malicious, and they are most likely instinctive. You're more likely to poke the ball away with two feet than one, or by trapping it between both feet, but that doesn't justify it. He needs to stop resorting to it, and referees need to punish them in a consistent manner. If players keep getting away with them, there is more temptation to do it.