dual           
adjective [ before noun ] uk/ˈdʒuː.əl/ us/ˈduː.əl/
C1 两部分的;二元的;双重的
with two parts, or combining two things
  This room has a dual purpose, serving as both a study and a dining room. 这个房间有两个用途,既作书房又当餐厅。 
 the dual role of chairman and chief executive 董事长和首席执行官的双重角色 
 dual controls (教练汽车的)双重控制,复式操纵
two sets of controls in a car, one for the person who is learning to drive and one for the teacher
 dual nationality/citizenship 双重国籍
the nationality of two countries at the same time
  She has dual British and American nationality. 她有英美双重国籍。 
 - More examples
 - Everything he says has a dual meaning.
 - Although she had the dual advantages of wealth and beauty, she did not have a happy life.
 - These painkillers have the dual strength of ibuprofen and codeine, for fast and effective relief.
 - A product with dual action - that kills bacteria and removes stains - is really the best.
 - The community hall was designed for dual use, by the school during school hours and by the public at other times.