dual

形容词 dual:

1. consisting of or involving two parts or components usually in pairs
   同义词：double, duple
2. having more than one decidedly dissimilar aspects or qualities-Frederick Harrison
   同义词：double, twofold, two-fold, treble, threefold, three-fold
3. a grammatical number category referring to two items or units as opposed to one item (singular) or more than two items (plural)

1. He has a dual interest in the football team; he's the trainer and his son plays for them.
   他跟那个足球队有双重关系，他是足球队教练，他的儿子是队员。
2. The road opened out into a dual carriageway.
   这条路已扩展成了复式车行道。
3. His dual role as a composer and a conductor made him very busy.
   他作曲家和指挥家的双重身份使他非常繁忙。
4. She has dual nationality.
   她有双重国籍。

du.al

adj.（形容词）
Composed of two usually like or complementary parts; double:
成双的：由两个通常相象或互补部分组成的；双的：
例句：
dual controls for pilot and copilot; a car with dual exhaust pipes.
飞行员与副飞行员复式操纵装置；有双排气管的汽车

Having a double character or purpose:
有双重特点或目的的：
例句：
a belief in the dual nature of reality.
认为现实具有二重本性的看法

Grammar Of, relating to, or being a number category that indicates two persons or things, as in Greek, Sanskrit, and Old English.
【语法】 双数的：指示两个人或物的数字范畴的，如希腊语，梵语与古英语中双数概念
n.Grammar （名词）【语法】
The dual number.
双数
An inflected form of a noun, adjective, pronoun, or verb used with two items or people.
双数格：用于两个物体或人时名词、形容词、代词或动词的曲折变化词形

来源：
Latin du3lis
拉丁语 du3lis
from duo [two] * see dwo-
源自 duo [二] *参见 dwo-

【引伸】
du.ally
adv.（副词）