1有一句寫:a pilot and his pessenger were killed when the small plane they were flying in got distracted while taking a selfie.這一句居然有兩連接詞，為何?

why not? 不但2個可以,3個,4個,5個都行.只是越來越不好讀,越來越醜就是了.
例子:

A pilot and his passenger were killed when the small plane they were flying in got distracted while taking a selfie before taking his final exam and after having twin babies.

 不但2個可以,3個,4個,5個都行

A pilot and his passenger were killed when the small plane (that) they were flying in got distracted while they were taking a selfie.
主要子句：A pilot and his passenger were killed.
副詞子句：when the small plane got distracted 修飾主句的述部，提供其時間點
形容詞子句：(that) they were flying修飾前面的名詞plane，連接詞that因是子句裡的受詞，所以可省略
副詞子句：while they were taking a selfie修飾副詞子句的述部，是供其時間點。they were必須標示出來，否則成了taking a selfie的是the small plane：小飛機在玩自拍？因為分詞構句邏輯上的主詞，是其修飾的句子的主詞時，才可省略，但本句不是）

「句焦點」是主要子句（有個飛行員和乘客死亡了），其餘次要的資料，以修飾語形態放在句中。

兩個連接詞（when/while）的用法無誤：用在副詞性質的子句上。

可以有很多個連接詞,用連接詞可以讓句子變得更通順,也能展現你的英文程度

英文口語甚至正式寫作
常是能省則省因此有幾
個連接詞或是省略主詞、動詞、名詞等都有可能

問得好，句子可以有兩個連接詞，只是問題連接的"前後文"是不是同一主角。

現在這句的第一個連接" XX+YY were killed when the plane got distracted"還可以肯定是"那飛機受到干擾。

但是第二個連接"while taking a selfie"句中沒有主詞，假如照句子的結構，while是連接前一句和後一句，那麼前一句的主詞是the small plane小型飛機，後句"自拍"的便是這"小型飛機"。

全句意思是：

一架小型飛機臨降落前正在自拍引致干擾導致機師和一名乘客被殺。

我認為比較正確的寫法應該是：

A pilot and his passenger were killed while taking a selfie got distracted and crashed the small plane they were flying in.

內容中got distracted應該是指那"控制"飛機的機師被干擾，而不是那飛機被干擾，除非"某行為"導致飛機的功能被干擾才可以寫成the small plane got distracted.

因此這句的基本結構和內容都是錯的。

或者從寫作的立場，這句其實是由讀者"意會"內容，而不是由作者正確地"描述內容"。

因為作者認為他自己在寫:
a pilot and his passenger were killed
while taking a selfie
+
when the small plane they were flying in got distracted by the selfie

關於你的問題，這裡有別人問
https://tw.answers.yahoo.com/question/index?qid=20091006000051KK00339

所以是可以兩個連接詞的。