The Kids Are All Right star joined the Frieze art fair on Randall's Island as part of a bizarre show by artist Rirkrit Tiravanija.
He spent two and a half hours cooking sausage sandwiches with gallery owner Gavin Brown, who was dressed in an identical outfit and sported a matching hairstyle and beard.
A source tells the New York Post's gossip column Page Six, "Mark and Gavin wore matching navy blue Dickies shirts and pants, and white aprons. And they made sausage sandwiches on hot plates for 2 1/2 hours. At first glance, you couldn't tell them apart."