“21” or “Twenty-one” (two of an item for the same person or table)

 
Newspapers.com
27 September 1981, The Herald Statesman (Yonkers, NY), “Counterwailing” by Dan Carlinsky, Suburdia Today, pg. 18, col. 4:
21. Two of an item for the same person or table. “A burger 21” means two hamburgers on one plate.