You can picture an old shotgun lying open on a table: not one tube but two, lying side by side. Everything this word does comes from that one picture. Two tubes means two of something joined into one object — which is how a name with two halves, or a plan with two aims, gets the word. And two tubes also means both can go off together, which is where the sense of great force comes from.
可以想成一把打開的舊獵槍放在桌上：不是一根管子，是兩根並排。這個字所有的用法都從這個畫面長出來。「兩根並排」表示兩個東西合成一個，所以兩節的姓、兩個目標的計畫都用得上它。而兩根管子也可以一起開火，所以它又帶出「力道加倍」的意思。
From the word’s history
The word is simply double- + barrelled, so its literal sense — having two barrels — is also its picture.
這個字就是 double-（兩個）加上 barrelled（有管子的），所以它的字面意思跟它的畫面是同一件事：有兩根管子。後面的用法都是把這個畫面借走。
Wiktionary, "double-barrelled"
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