Sure, there are likely fanfics on this. In the 'Game of Thrones' universe, Daenerys and Jon were two of the most beloved characters. Their relationship was complex and full of potential. Fanfic authors often take these elements and run with them. The idea of them having twins while pregnant can be a way to explore family, legacy, and the future in a fictional world that was left somewhat open - ended in the show. Some fanfics may focus on the pregnancy itself, how they deal with the news, and how it affects their relationship with other characters.
There could be. Many fanfiction writers like to explore the relationships between characters in new and unexpected ways. Since Daenerys and Jon were important characters in 'Game of Thrones' and had a relationship in the show, it's a popular topic for fanfic writers to expand on. They might come up with the idea of them having twins and the whole pregnancy journey to add more drama and romance to the story.