The fan base loves these characters so much that seeing them interact in new and unexpected ways is always a treat. The relationship between them can be explored in so many different ways, from friendship to rivalry. And the unique art styles and interpretations that often come with these fanfictions add to the overall appeal. There are also some really emotional stories that can be told about them finding common ground or learning from each other, which really tugs at the heartstrings of the readers.
There's also 'Sibling Rivalry Reimagined' which delves deep into how their personalities from their respective universes interact. It has some really interesting character development for both Sanses. It shows how their differences can lead to both conflict and unexpected understanding. The author does a great job of portraying their emotions and the way they communicate, which is quite different from the original game's interactions.
Well, it could be really interesting. Maybe it shows a unique interaction between Chara from Storyshift and Sans from Undertale. Their personalities are quite different. Chara can be rather complex and Sans is known for his laid - back yet perceptive nature. So in the fanfiction, there might be a lot of unexpected exchanges and development of their relationship.
From Undertale, characters like Sans or Frisk could be main characters too. Sans, with his laid - back yet intelligent nature, could provide a great contrast to Error Sans. And Frisk, being the human with the ability to reset and change the course of events, would be crucial in any plot involving the two different universes colliding.
Maybe it's about the characters from Undertale trying to understand Error Sans. Error Sans is a complex character with his own motives and backstory. In the fanfiction, the Undertale characters could be trying to figure out why he is causing disruptions and if there's a way to coexist with him. This could involve a lot of character development and exploration of different moral stances.
Another plotline could be that there is a rift in the multiverse, and Chara and Sans are brought together to fix it. Along the way, they discover things about each other's universes and themselves, and in the end, they manage to close the rift and return to their respective worlds with a new understanding of the multiverse.
The contrast between the two characters. Underswap Sans is more laid - back and friendly compared to Underfell Sans who is more rough and edgy. When they interact in fanfictions, it creates a lot of interesting dynamics, like how they might clash or gradually understand each other.
Often, the cuteness of the baby form is a key feature. Despite being from the Underfell universe which is usually more edgy, the baby version brings out a different side.
You can start by looking on Archive of Our Own (AO3). It has a large collection of fanfictions for various fandoms, including the Undertale AU crossovers like Underswap Sans x Underfell Sans. Another place could be Fanfiction.net, though the quality may vary more there. Additionally, Tumblr sometimes has users sharing or recommending such fanfictions in the Undertale fandom tags.
One popular 'baby underfell sans fanfiction' is 'The Tiny Terror'. It has an interesting take on the character in a baby form, with unique plot twists and character interactions.