Mixed slabs are a popular feature in slab mods, but I've noticed double slab mods are scarce for Fabric players (several of these mods aren't up to date, or skip versions like 1.20.1 and 1.21).

KleeSlabs is a mod focused on allowing individual breaking of slabs and has been for almost 10 years. Mixed slabs is an entirely different feature and a whole new mod on its own. It's way out of scope for this mod.