A double conjunction in German is a combination of two conjunctions that work together to connect parts of a sentence or multiple sentences. They come in many forms, but what they all have in common is that they help connect thoughts and make speech flow more smoothly. For example, in the conjunction “nicht nur.... sondern auch”, the first word adds information and the second word emphasizes that it is something important or unexpected.
Why are they needed? They help to make speech more expressive, not by repeating the same thing, but by adding variety. Instead of saying “Ich habe einen Hund und eine Katze” (I have a dog and a cat), you can say “Ich habe nicht nur einen Hund, sondern auch eine Katze” (I have not only a dog but also a cat). This sounds more interesting and a bit more colorful!
Double conjunctions help you express exactly how you perceive the relationship between ideas: for example, opposition, addition or choice. They're like superpowers for your language!