Okay, trying to find a great platform for Pico x Darnell crossover stories feels a bit like looking for a very specific kind of collectible figure at a huge convention. You know it's out there, but you have to know which booths are run by the real enthusiasts. For these characters from 'Friday Night Funkin'' and 'Pico’s School', the central hub is absolutely Archive of Our Own, or AO3. The tagging system over there is a godsend for niche crossovers; you can filter for fandom blends, specific relationship tags, and all sorts of content warnings or tropes. The community around FNF and Newgrounds-era content is incredibly active on AO3, so you'll find a range of stories, from quick, silly one-shots exploring how these two wildly different characters might interact, to more involved plots that dive into their shared history of, well, surviving violent scenarios. The quality can vary, as with any archive, but the dedication of the writers and the feedback culture means the top works are often polished and genuinely creative.
Sometimes, though, you might stumble onto something great in the wilds of Tumblr. Writers will often post shorter drabbles or headcanon threads about the pairing directly on their blogs, and those can be fun, immediate bursts of creativity. The trick is using the right tags in your search (#pico x darnell, #friday night funkin fanfiction, etc.), as Tumblr's search isn't always the most reliable. Wattpad hosts some stories too, but in my experience, the tagging and organization isn't as precise for this specific crossover, so you'll likely wade through more unrelated content. Really, if you want the deepest well of stories that take these characters seriously (or play with them hilariously), AO3 is where you need to set up camp. I've lost whole afternoons there, just following the trail of related works people have bookmarked.