You're hitting on a crossover niche that's actually pretty active! Those two have such complimentary energy—Chigiri's meticulous grace and Kunigami's raw power—that they naturally slot into other sports or action-heavy worlds. My absolute favorite right place to dig is AO3; filtering by the 'Chigiri Hyoma/Kunigami Rensuke' tag, then adding 'Crossover' to the additional tags filter gets you the dedicated stuff. A surprising number of writers throw them into 'Haikyuu!!' universes, which makes sense given the volleyball framing. There's this one longfic, 'Crosscourt', that reimagines them as rival aces at different high schools, and the tension is phenomenal.
Don't sleep on smaller fandoms either. I've seen them pop up in 'Free!' AUs as competitive swimmers, or even in fantasy settings like 'Jujutsu Kaisen' where their Blue Lock abilities get reworked as cursed techniques. Tumblr can be a good discovery tool if you search the ship tag and follow reblog chains, though the actual stories usually link back to AO3. The key is patience; since it's a crossover, updates aren't as frequent as pure 'Blue Lock' fic, but the ones that exist are often really thoughtful about merging the worlds.