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.
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 entry for double-barrelled, etymology: "From double- + barrelled." (kaikki extract kaikki:3f6f69a190036ae4:1040812425:9177)
Two branches
兩根並排：兩個部分合成一個
這一組守著字面的畫面：重點在「數量是兩個」。從真的兩根槍管，到名字的兩節、計畫的兩條路，都是同一件事換了材料。
adjective · C1
Used about a gun that has two barrels — the two long tubes that the shot comes out of — set side by side or one above the other, so it can fire twice before it is loaded again.
雙管的
兩管齊發：力道加倍
同一把槍，換一個角度看：不是數管子，而是聽它一起開火的聲音。所以這一支的重點從「兩個」變成「很猛」。
adjective · C1
Used about something that comes at you with a lot of force — a very strong attack, criticism, or effort — as if both barrels of a gun went off at the same time.
火力十足的