A Double Sentence is a sentence in which there will be two subjects, two verbs and two predicates. A Double Sentence is one that could be split into two other sentences.
1. The sun rose and the fog dispersed.
2. The sun rose.
3. The fog dispersed.
Here each co-ordinate part is a Simple sentence.
1. We asked her where she had lost the ring, but she refused to answer.
2. We asked her where she had lost the ring.
3. She refused to answer.
Here the first co-ordinate part is a Complex sentence, while the second part is a Simple sentence.
1. I say what I mean, and I mean what I say.
2. I say what I mean.
3. I mean what I say.
Here each co-ordinate part is a Complex sentence.
All the Double Sentences are called Multiple Sentences. But all the Multiple Sentences are not called as Double Sentences.