It’s not the duck that’s a problem. That would his 15 mates and extended family all going the same way.

Look in any English language dictionary. Show me an entry that states a couple is more than two

I‘ll wait.

Wrong. A couple is two exactly. After the wedding: Oh look at the happy couple. There aren’t 3 or 4 people standing there, 2 people are standing there. A couple.

To couple train carriages together means to attach two carriages together. There are more carriages behind that one, but they were all individually coupled together.