Do Scout and Dill stay together to fulfill their promised marriage in the future?
This is a good question. The answer in short is we do not know.
In a humorous section of the book, Scout and Dill become good friends. Within this context, a crush develops as Dill promises to marry Scout. We read of this in chapter 12. In a letter Dill apologizes that he could not come to Maycomb, but he reminds Scout that he will be back to fulfill his promise of marriage. Here is the quote:
Dill concluded by saying he would love me forever and not to worry, he would come get me and marry me as soon as he got enough money together, so please write. The fact that I had a permanent fiancé was little compensation for his absence:
We do not know what happens later on, because the book does not trace the lives of Scout and Dill. In fact, if we look at the book from a temporal point of view, the book starts when Scout is six and the book ends when she is nine. That said, one does hope that Dill and Scout do get together.