Honestly, the Whitty and Ruv crossover scene is way smaller than I expected, given how popular both characters are individually in their own spheres. You'll find most of them on dedicated modding and indie game fanfiction hubs rather than the big archives. There's a solid little niche on Fanfiction.net if you filter by the 'Friday Night Funkin'' fandom and cross-reference with mod tags, but the tagging can be a mess—sometimes they're just tagged 'Whitty' and you have to spot Ruv in the summary.
I stumbled across one called 'Resonant Static' that does a pretty interesting take, framing their meeting as less of a direct battle and more of two beings with volatile powers recognizing a similar kind of instability in each other. The writer really leaned into the sonic/audio-visual distortion aspects of both characters. Another one, 'Cadence Clash,' goes full fight scene, which is fun if you just want to see them try to out-blast each other. The character voices in that one are a bit off, but the energy is there.
It's a pairing that feels like it should have more, maybe because their core conflicts are so internal. A lot of writers seem to default to just throwing them into a versus scenario without digging into that.